Compressed Image File Formats/CompressedImageFileFormatsJohnMiano.iso/mac/Specs/png-1.1-pdg.txt

Text File | 1999-02-12 | 249KB | 5,183 lines

PNG Development Group                        G. Randers-Pehrson, et. al.
PNG 1.1                                                     January 1999


       PNG (Portable Network Graphics) Specification, Version 1.1

For list of authors, see Credits (Chapter 19).

Status of this Document

   This is a revision of the PNG 1.0 specification, which has been
   published as RFC-2083 and as a W3C Recommendation.  The revision has
   been released by the PNG Development Group but has not been approved
   by any standards body.

Abstract

   This document describes PNG (Portable Network Graphics), an
   extensible file format for the lossless, portable, well-compressed
   storage of raster images.  PNG provides a patent-free replacement for
   GIF and can also replace many common uses of TIFF.  Indexed-color,
   grayscale, and truecolor images are supported, plus an optional alpha
   channel.  Sample depths range from 1 to 16 bits.

   PNG is designed to work well in online viewing applications, such as
   the World Wide Web, so it is fully streamable with a progressive
   display option.  PNG is robust, providing both full file integrity
   checking and simple detection of common transmission errors.  Also,
   PNG can store gamma and chromaticity data for improved color matching
   on heterogeneous platforms.

   This specification defines the Internet Media Type "image/png".

Table of Contents

   1. Introduction
   2. Data Representation
      2.1. Integers and byte order
      2.2. Color values
      2.3. Image layout
      2.4. Alpha channel
      2.5. Filtering
      2.6. Interlaced data order
      2.7. Gamma correction
      2.8. Text strings
   3. File Structure
      3.1. PNG file signature
      3.2. Chunk layout
      3.3. Chunk naming conventions
      3.4. CRC algorithm
   4. Chunk Specifications
      4.1. Critical chunks
          4.1.1. IHDR Image header
          4.1.2. PLTE Palette
          4.1.3. IDAT Image data
          4.1.4. IEND Image trailer
      4.2. Ancillary chunks
          4.2.1. bKGD Background color
          4.2.2. cHRM Primary chromaticities and white point
          4.2.3. gAMA Image gamma
          4.2.4. hIST Palette histogram
          4.2.5. iCCP Embedded ICC profile
          4.2.6. pHYs Physical pixel dimensions
          4.2.7. sBIT Significant bits
          4.2.8. sPLT Suggested palette
          4.2.9. sRGB Standard RGB color space
          4.2.10. tEXt Textual data
          4.2.11. tIME Image last-modification time
          4.2.12. tRNS Transparency
          4.2.13. zTXt Compressed textual data
      4.3. Summary of standard chunks
      4.4. Additional chunk types
   5. Deflate/Inflate Compression
   6. Filter Algorithms
      6.1. Filter types
      6.2. Filter type 0: None
      6.3. Filter type 1: Sub
      6.4. Filter type 2: Up
      6.5. Filter type 3: Average
      6.6. Filter type 4: Paeth
   7. Chunk Ordering Rules
      7.1. Behavior of PNG editors
      7.2. Ordering of ancillary chunks
      7.3. Ordering of critical chunks
   8. Miscellaneous Topics
      8.1. File name extension
      8.2. Internet media type
      8.3. Macintosh file layout
      8.4. Multiple-image extension
      8.5. Security considerations
   9. Recommendations for Encoders
      9.1. Sample depth scaling
      9.2. Encoder gamma handling
      9.3. Encoder color handling
      9.4. Alpha channel creation
      9.5. Suggested palettes
      9.6. Filter selection
      9.7. Text chunk processing
      9.8. Use of private chunks
      9.9. Private type and method codes
   10. Recommendations for Decoders
      10.1. Error checking
      10.2. Pixel dimensions
      10.3. Truecolor image handling
      10.4. Sample depth rescaling
      10.5. Decoder gamma handling
      10.6. Decoder color handling
      10.7. Background color
      10.8. Alpha channel processing
      10.9. Progressive display
      10.10. Suggested-palette and histogram usage
      10.11. Text chunk processing
   11. Glossary
   12. Appendix: Rationale
      12.1. Why a new file format?
      12.2. Why these features?
      12.3. Why not these features?
      12.4. Why not use format X?
      12.5. Byte order
      12.6. Interlacing
      12.7. Why gamma?
      12.8. Non-premultiplied alpha
      12.9. Filtering
      12.10. Text strings
      12.11. PNG file signature
      12.12. Chunk layout
      12.13. Chunk naming conventions
      12.14. Palette histograms
   13. Appendix: Gamma Tutorial
      13.1. Nonlinear transfer functions
      13.2. Combining exponents
      13.3. End-to-end exponent
      13.4. CRT exponent
      13.5. Gamma correction
      13.6. Benefits of gamma encoding
      13.7. General gamma handling
      13.8. Some specific examples
      13.9. Video camera transfer functions
      13.10. Further reading
   14. Appendix: Color Tutorial
      14.1. About chromaticity
      14.2. The problem of color
      14.3. Device-dependent color
      14.4. Device-independent color
      14.5. Calibrated device-dependent color
      14.6. Chromaticity and luminance
      14.7. Characterizing computer monitors
      14.8. Uses for XYZ
      14.9. Converting between RGB and XYZ
      14.10. Device gamut
      14.11. Further reading
   15. Appendix: Sample CRC Code
   16. Appendix: Online Resources
   17. Appendix: Revision History
   18. References
   19. Credits

1. Introduction

   The Portable Network Graphics (PNG) format provides a portable,
   legally unencumbered, well-compressed, well-specified standard for
   lossless bitmapped image files.

   Although the initial motivation for developing PNG was to replace GIF
   (CompuServe's Graphics Interchange Format), the design provides some
   useful new features not available in GIF, with minimal cost to
   developers.

   GIF features retained in PNG include:

       * Indexed-color images of up to 256 colors.

       * Streamability: files can be read and written serially, thus
         allowing the file format to be used as a communications
         protocol for on-the-fly generation and display of images.

       * Progressive display: a suitably prepared image file can be
         displayed as it is received over a communications link,
         yielding a low-resolution image very quickly followed by
         gradual improvement of detail.

       * Transparency: portions of the image can be marked as
         transparent, creating the effect of a non-rectangular image.

       * Ancillary information: textual comments and other data can be
         stored within the image file.

       * Complete hardware and platform independence.

       * Effective, 100% lossless compression.

   Important new features of PNG, not available in GIF, include:

       * Truecolor images of up to 48 bits per pixel.

       * Grayscale images of up to 16 bits per pixel.

       * Full alpha channel (general transparency masks).

       * Image gamma information, which supports automatic display of
         images with correct brightness/contrast regardless of the
         machines used to originate and display the image.

       * Reliable, straightforward detection of file corruption.

       * Faster initial presentation in progressive display mode.

   PNG is designed to be:

       * Simple and portable: developers should be able to implement PNG
         easily.

       * Legally unencumbered: to the best knowledge of the PNG authors,
         no algorithms under legal challenge are used.  (Some
         considerable effort has been spent to verify this.)

       * Well compressed: both indexed-color and truecolor images are
         compressed as effectively as in any other widely used lossless
         format, and in most cases more effectively.

       * Interchangeable: any standard-conforming PNG decoder must read
         all conforming PNG files.

       * Flexible: the format allows for future extensions and private
         add-ons, without compromising interchangeability of basic PNG.

       * Robust: the design supports full file integrity checking as
         well as simple, quick detection of common transmission errors.

   The main part of this specification gives the definition of the file
   format and recommendations for encoder and decoder behavior.  An
   appendix gives the rationale for many design decisions.  Although the
   rationale is not part of the formal specification, reading it can
   help implementors understand the design.  Cross-references in the
   main text point to relevant parts of the rationale.  Additional
   appendixes, also not part of the formal specification, provide
   tutorials on gamma and color theory as well as other supporting
   material.

   The words "must", "required", "should", "recommended", "may", and
   "optional" in this document are to be interpreted as described in
   [RFC-2119], which is consistent with their plain English meanings.
   The word "can" carries the same force as "may".

   See Rationale: Why a new file format? (Section 12.1), Why these
   features? (Section 12.2), Why not these features? (Section 12.3), Why
   not use format X? (Section 12.4).

   Pronunciation

      PNG is pronounced "ping".

2. Data Representation

   This chapter discusses basic data representations used in PNG files,
   as well as the expected representation of the image data.

   2.1. Integers and byte order

      All integers that require more than one byte must be in network
      byte order: the most significant byte comes first, then the less
      significant bytes in descending order of significance (MSB LSB for
      two-byte integers, B3 B2 B1 B0 for four-byte integers).  The
      highest bit (value 128) of a byte is numbered bit 7; the lowest
      bit (value 1) is numbered bit 0.  Values are unsigned unless
      otherwise noted.  Values explicitly noted as signed are
      represented in two's complement notation.

      Unless otherwise stated, four-byte unsigned integers are limited
      to the range 0 to (2^31)-1 to accommodate languages that have
      difficulty with unsigned four-byte values.  Similarly, four-byte
      signed integers are limited to the range -((2^31)-1) to (2^31)-1
      to accommodate languages that have difficulty with the value
      -2^31.

      See Rationale: Byte order (Section 12.5).

   2.2. Color values

      Colors can be represented by either grayscale or RGB (red, green,
      blue) sample data.  Grayscale data represents luminance; RGB data
      represents calibrated color information (if the cHRM chunk is
      present) or uncalibrated device-dependent color (if cHRM is
      absent).  All color values range from zero (representing black) to
      most intense at the maximum value for the sample depth.  Note that
      the maximum value at a given sample depth is (2^sampledepth)-1,
      not 2^sampledepth.

      Sample values are not necessarily proportional to light intensity;
      the gAMA chunk specifies the relationship between sample values
      and display output intensity, and viewers are strongly encouraged
      to compensate properly.  See Gamma correction (Section 2.7).

      Source data with a precision not directly supported in PNG (for
      example, 5 bit/sample truecolor) must be scaled up to the next
      higher supported bit depth.  This scaling is reversible with no
      loss of data, and it reduces the number of cases that decoders
      have to cope with.  See Recommendations for Encoders: Sample depth
      scaling (Section 9.1) and Recommendations for Decoders: Sample
      depth rescaling (Section 10.4).

   2.3. Image layout

      Conceptually, a PNG image is a rectangular pixel array, with
      pixels appearing left-to-right within each scanline, and scanlines
      appearing top-to-bottom.  (For progressive display purposes, the
      data may actually be transmitted in a different order; see
      Interlaced data order, Section 2.6.) The size of each pixel is
      determined by the bit depth, which is the number of bits per
      sample in the image data.

      Three types of pixel are supported:

          * An indexed-color pixel is represented by a single sample
            that is an index into a supplied palette.  The image bit
            depth determines the maximum number of palette entries, but
            not the color precision within the palette.

          * A grayscale pixel is represented by a single sample that is
            a grayscale level, where zero is black and the largest value
            for the bit depth is white.

          * A truecolor pixel is represented by three samples: red (zero
            = black, max = red) appears first, then green (zero = black,
            max = green), then blue (zero = black, max = blue).  The bit
            depth specifies the size of each sample, not the total pixel
            size.

      Optionally, grayscale and truecolor pixels can also include an
      alpha sample, as described in the next section.

      Pixels are always packed into scanlines with no wasted bits
      between pixels.  Pixels smaller than a byte never cross byte
      boundaries; they are packed into bytes with the leftmost pixel in
      the high-order bits of a byte, the rightmost in the low-order
      bits.  Permitted bit depths and pixel types are restricted so that
      in all cases the packing is simple and efficient.

      PNG permits multi-sample pixels only with 8- and 16-bit samples,
      so multiple samples of a single pixel are never packed into one
      byte.  All 16-bit samples are stored in network byte order (MSB
      first).

      Scanlines always begin on byte boundaries.  When pixels have fewer
      than 8 bits and the scanline width is not evenly divisible by the
      number of pixels per byte, the low-order bits in the last byte of
      each scanline are wasted.  The contents of these wasted bits are
      unspecified.

      An additional "filter-type" byte is added to the beginning of
      every scanline (see Filtering, Section 2.5). The filter-type byte
      is not considered part of the image data, but it is included in
      the datastream sent to the compression step.

   2.4. Alpha channel

      An alpha channel, representing transparency information on a per-
      pixel basis, can be included in grayscale and truecolor PNG
      images.

      An alpha value of zero represents full transparency, and a value
      of (2^bitdepth)-1 represents a fully opaque pixel.  Intermediate
      values indicate partially transparent pixels that can be combined
      with a background image to yield a composite image.  (Thus, alpha
      is really the degree of opacity of the pixel.  But most people
      refer to alpha as providing transparency information, not opacity
      information, and we continue that custom here.)

      Alpha channels can be included with images that have either 8 or
      16 bits per sample, but not with images that have fewer than 8
      bits per sample.  Alpha samples are represented with the same bit
      depth used for the image samples.  The alpha sample for each pixel
      is stored immediately following the grayscale or RGB samples of
      the pixel.

      The color values stored for a pixel are not affected by the alpha
      value assigned to the pixel.  This rule is sometimes called
      "unassociated" or "non-premultiplied" alpha.  (Another common
      technique is to store sample values premultiplied by the alpha
      fraction; in effect, such an image is already composited against a
      black background.  PNG does not use premultiplied alpha.)

      Transparency control is also possible without the storage cost of
      a full alpha channel.  In an indexed-color image, an alpha value
      can be defined for each palette entry.  In grayscale and truecolor
      images, a single pixel value can be identified as being
      "transparent".  These techniques are controlled by the tRNS
      ancillary chunk type.

      If no alpha channel nor tRNS chunk is present, all pixels in the
      image are to be treated as fully opaque.

      Viewers can support transparency control partially, or not at all.

      See Rationale: Non-premultiplied alpha (Section 12.8),
      Recommendations for Encoders: Alpha channel creation (Section
      9.4), and Recommendations for Decoders: Alpha channel processing
      (Section 10.8).

   2.5. Filtering

      PNG allows the image data to be filtered before it is compressed.
      Filtering can improve the compressibility of the data.  The filter
      step itself does not reduce the size of the data.  All PNG filters
      are strictly lossless.

      PNG defines several different filter algorithms, including "None"
      which indicates no filtering.  The filter algorithm is specified
      for each scanline by a filter-type byte that precedes the filtered
      scanline in the precompression datastream.  An intelligent encoder
      can switch filters from one scanline to the next.  The method for
      choosing which filter to employ is up to the encoder.

      See Filter Algorithms (Chapter 6) and Rationale: Filtering
      (Section 12.9).

   2.6. Interlaced data order

      A PNG image can be stored in interlaced order to allow progressive
      display.  The purpose of this feature is to allow images to "fade
      in" when they are being displayed on-the-fly.  Interlacing
      slightly expands the file size on average, but it gives the user a
      meaningful display much more rapidly.  Note that decoders are
      required to be able to read interlaced images, whether or not they
      actually perform progressive display.

      With interlace method 0, pixels are stored sequentially from left
      to right, and scanlines sequentially from top to bottom (no
      interlacing).

      Interlace method 1, known as Adam7 after its author, Adam M.
      Costello, consists of seven distinct passes over the image.  Each
      pass transmits a subset of the pixels in the image.  The pass in
      which each pixel is transmitted is defined by replicating the
      following 8-by-8 pattern over the entire image, starting at the
      upper left corner:

         1 6 4 6 2 6 4 6
         7 7 7 7 7 7 7 7
         5 6 5 6 5 6 5 6
         7 7 7 7 7 7 7 7
         3 6 4 6 3 6 4 6
         7 7 7 7 7 7 7 7
         5 6 5 6 5 6 5 6
         7 7 7 7 7 7 7 7

      Within each pass, the selected pixels are transmitted left to
      right within a scanline, and selected scanlines sequentially from
      top to bottom.  For example, pass 2 contains pixels 4, 12, 20,
      etc. of scanlines 0, 8, 16, etc. (numbering from 0,0 at the upper
      left corner).  The last pass contains the entirety of scanlines 1,
      3, 5, etc.

      The data within each pass is laid out as though it were a complete
      image of the appropriate dimensions.  For example, if the complete
      image is 16 by 16 pixels, then pass 3 will contain two scanlines,
      each containing four pixels.  When pixels have fewer than 8 bits,
      each such scanline is padded as needed to fill an integral number
      of bytes (see Image layout, Section 2.3). Filtering is done on
      this reduced image in the usual way, and a filter-type byte is
      transmitted before each of its scanlines (see Filter Algorithms,
      Chapter 6). Notice that the transmission order is defined so that
      all the scanlines transmitted in a pass will have the same number
      of pixels; this is necessary for proper application of some of the
      filters.

      Caution: If the image contains fewer than five columns or fewer
      than five rows, some passes will be entirely empty.  Encoders and
      decoders must handle this case correctly.  In particular, filter-
      type bytes are associated only with nonempty scanlines; no
      filter-type bytes are present in an empty pass.

      See Rationale: Interlacing (Section 12.6) and Recommendations for
      Decoders: Progressive display (Section 10.9).

   2.7. Gamma correction

      PNG images can specify, via the gAMA chunk, the power function
      relating the desired display output with the image samples.
      Display programs are strongly encouraged to use this information,
      plus information about the display system they are using, to
      present the image to the viewer in a way that reproduces what the
      image's original author saw as closely as possible.  See Gamma
      Tutorial (Chapter 13) if you aren't already familiar with gamma
      issues.

      Gamma correction is not applied to the alpha channel, if any.
      Alpha samples always represent a linear fraction of full opacity.

      For high-precision applications, the exact chromaticity of the RGB
      data in a PNG image can be specified via the cHRM chunk, allowing
      more accurate color matching than gamma correction alone will
      provide.  If the RGB data conforms to the sRGB specification
      [sRGB], this can be indicated with the sRGB chunk, enabling even
      more accurate reproduction.  Alternatively, the iCCP chunk can be
      used to embed an ICC profile [ICC] containing detailed color space
      information.  See Color Tutorial (Chapter 14) if you aren't
      already familiar with color representation issues.

      See Rationale: Why gamma? (Section 12.7), Recommendations for
      Encoders: Encoder gamma handling (Section 9.2), and
      Recommendations for Decoders: Decoder gamma handling (Section
      10.5).

   2.8. Text strings

      A PNG file can store text associated with the image, such as an
      image description or copyright notice.  Keywords are used to
      indicate what each text string represents.

      ISO/IEC 8859-1 (Latin-1) is the character set recommended for use
      in text strings [ISO/IEC-8859-1]. It is a superset of 7-bit ASCII.

      Character codes not defined in Latin-1 should not be used, because
      they have no platform-independent meaning.  If a non-Latin-1 code
      does appear in a PNG text string, its interpretation will vary
      across platforms and decoders.  Some systems might not even be
      able to display all the characters in Latin-1, but most modern
      systems can.

      Provision is also made for the storage of compressed text.

      See Rationale: Text strings (Section 12.10).

3. File Structure

   A PNG file consists of a PNG signature followed by a series of
   chunks.  This chapter defines the signature and the basic properties
   of chunks.  Individual chunk types are discussed in the next chapter.

   3.1. PNG file signature

      The first eight bytes of a PNG file always contain the following
      (decimal) values:

         137 80 78 71 13 10 26 10

      This signature indicates that the remainder of the file contains a
      single PNG image, consisting of a series of chunks beginning with
      an IHDR chunk and ending with an IEND chunk.

      See Rationale: PNG file signature (Section 12.11).

   3.2. Chunk layout

      Each chunk consists of four parts:

      Length
         A 4-byte unsigned integer giving the number of bytes in the
         chunk's data field.  The length counts only the data field, not
         itself, the chunk type code, or the CRC.  Zero is a valid
         length.  Although encoders and decoders should treat the length
         as unsigned, its value must not exceed (2^31)-1 bytes.

      Chunk Type
         A 4-byte chunk type code.  For convenience in description and
         in examining PNG files, type codes are restricted to consist of
         uppercase and lowercase ASCII letters (A-Z and a-z, or 65-90
         and 97-122 decimal).  However, encoders and decoders must treat
         the codes as fixed binary values, not character strings.  For
         example, it would not be correct to represent the type code
         IDAT by the EBCDIC equivalents of those letters.  Additional
         naming conventions for chunk types are discussed in the next
         section.

      Chunk Data
         The data bytes appropriate to the chunk type, if any.  This
         field can be of zero length.

      CRC
         A 4-byte CRC (Cyclic Redundancy Check) calculated on the
         preceding bytes in the chunk, including the chunk type code and
         chunk data fields, but not including the length field.  The CRC
         is always present, even for chunks containing no data.  See CRC
         algorithm (Section 3.4).

      The chunk data length can be any number of bytes up to the
      maximum; therefore, implementors cannot assume that chunks are
      aligned on any boundaries larger than bytes.

      Chunks can appear in any order, subject to the restrictions placed
      on each chunk type.  (One notable restriction is that IHDR must
      appear first and IEND must appear last; thus the IEND chunk serves
      as an end-of-file marker.)  Multiple chunks of the same type can
      appear, but only if specifically permitted for that type.

      See Rationale: Chunk layout (Section 12.12).

   3.3. Chunk naming conventions

      Chunk type codes are assigned so that a decoder can determine some
      properties of a chunk even when it does not recognize the type
      code.  These rules are intended to allow safe, flexible extension
      of the PNG format, by allowing a decoder to decide what to do when
      it encounters an unknown chunk.  The naming rules are not normally
      of interest when the decoder does recognize the chunk's type.

      Four bits of the type code, namely bit 5 (value 32) of each byte,
      are used to convey chunk properties.  This choice means that a
      human can read off the assigned properties according to whether
      each letter of the type code is uppercase (bit 5 is 0) or
      lowercase (bit 5 is 1).  However, decoders should test the
      properties of an unknown chunk by numerically testing the
      specified bits; testing whether a character is uppercase or
      lowercase is inefficient, and even incorrect if a locale-specific
      case definition is used.

      It is worth noting that the property bits are an inherent part of
      the chunk name, and hence are fixed for any chunk type.  Thus,
      TEXT and Text would be unrelated chunk type codes, not the same
      chunk with different properties.  Decoders must recognize type
      codes by a simple four-byte literal comparison; it is incorrect to
      perform case conversion on type codes.

      The semantics of the property bits are:

      Ancillary bit: bit 5 of first byte
         0 (uppercase) = critical, 1 (lowercase) = ancillary.

         Chunks that are not strictly necessary in order to meaningfully
         display the contents of the file are known as "ancillary"
         chunks.  A decoder encountering an unknown chunk in which the
         ancillary bit is 1 can safely ignore the chunk and proceed to
         display the image.  The time chunk (tIME) is an example of an
         ancillary chunk.

         Chunks that are necessary for successful display of the file's
         contents are called "critical" chunks.  A decoder encountering
         an unknown chunk in which the ancillary bit is 0 must indicate
         to the user that the image contains information it cannot
         safely interpret.  The image header chunk (IHDR) is an example
         of a critical chunk.

      Private bit: bit 5 of second byte
         0 (uppercase) = public, 1 (lowercase) = private.

         A public chunk is one that is part of the PNG specification or
         is registered in the list of PNG special-purpose public chunk
         types.  Applications can also define private (unregistered)
         chunks for their own purposes.  The names of private chunks
         must have a lowercase second letter, while public chunks will
         always be assigned names with uppercase second letters.  Note
         that decoders do not need to test the private-chunk property
         bit, since it has no functional significance; it is simply an
         administrative convenience to ensure that public and private
         chunk names will not conflict.  See Additional chunk types
         (Section 4.4), and Recommendations for Encoders: Use of private
         chunks (Section 9.8).

      Reserved bit: bit 5 of third byte
         Must be 0 (uppercase) in files conforming to this version of
         PNG.

         The significance of the case of the third letter of the chunk
         name is reserved for possible future expansion.  At the present
         time all chunk names must have uppercase third letters.
         (Decoders should not complain about a lowercase third letter,
         however, as some future version of the PNG specification could
         define a meaning for this bit.  It is sufficient to treat a
         chunk with a lowercase third letter in the same way as any
         other unknown chunk type.)

      Safe-to-copy bit: bit 5 of fourth byte
         0 (uppercase) = unsafe to copy, 1 (lowercase) = safe to copy.

         This property bit is not of interest to pure decoders, but it
         is needed by PNG editors (programs that modify PNG files).
         This bit defines the proper handling of unrecognized chunks in
         a file that is being modified.

         If a chunk's safe-to-copy bit is 1, the chunk may be copied to
         a modified PNG file whether or not the software recognizes the
         chunk type, and regardless of the extent of the file
         modifications.

         If a chunk's safe-to-copy bit is 0, it indicates that the chunk
         depends on the image data.  If the program has made any changes
         to critical chunks, including addition, modification, deletion,
         or reordering of critical chunks, then unrecognized unsafe
         chunks must not be copied to the output PNG file.  (Of course,
         if the program does recognize the chunk, it can choose to
         output an appropriately modified version.)

         A PNG editor is always allowed to copy all unrecognized chunks
         if it has only added, deleted, modified, or reordered ancillary
         chunks.  This implies that it is not permissible for ancillary
         chunks to depend on other ancillary chunks.

         PNG editors that do not recognize a critical chunk must report
         an error and refuse to process that PNG file at all.  The
         safe/unsafe mechanism is intended for use with ancillary
         chunks.  The safe-to-copy bit will always be 0 for critical
         chunks.

         Rules for PNG editors are discussed further in Chunk Ordering
         Rules (Chapter 7).

      For example, the hypothetical chunk type name bLOb has the
      property bits:

         bLOb  <-- 32 bit chunk type code represented in text form
         ||||
         |||+- Safe-to-copy bit is 1 (lowercase letter; bit 5 is 1)
         ||+-- Reserved bit is 0     (uppercase letter; bit 5 is 0)
         |+--- Private bit is 0      (uppercase letter; bit 5 is 0)
         +---- Ancillary bit is 1    (lowercase letter; bit 5 is 1)

      Therefore, this name represents an ancillary, public, safe-to-copy
      chunk.

      See Rationale: Chunk naming conventions (Section 12.13).

   3.4. CRC algorithm

      Chunk CRCs are calculated using standard CRC methods with pre and
      post conditioning, as defined by ISO 3309 [ISO-3309] or ITU-T V.42
      [ITU-T-V42]. The CRC polynomial employed is

         x^32+x^26+x^23+x^22+x^16+x^12+x^11+x^10+x^8+x^7+x^5+x^4+x^2+x+1

      The 32-bit CRC register is initialized to all 1's, and then the
      data from each byte is processed from the least significant bit
      (1) to the most significant bit (128).  After all the data bytes
      are processed, the CRC register is inverted (its ones complement
      is taken).  This value is transmitted (stored in the file) MSB
      first.  For the purpose of separating into bytes and ordering, the
      least significant bit of the 32-bit CRC is defined to be the
      coefficient of the x^31 term.

      Practical calculation of the CRC always employs a precalculated
      table to greatly accelerate the computation.  See Sample CRC Code
      (Chapter 15).

4. Chunk Specifications

   This chapter defines the standard types of PNG chunks.

   4.1. Critical chunks

      All implementations must understand and successfully render the
      standard critical chunks.  A valid PNG image must contain an IHDR
      chunk, one or more IDAT chunks, and an IEND chunk.

      4.1.1. IHDR Image header

         The IHDR chunk must appear FIRST.  It contains:

            Width:              4 bytes
            Height:             4 bytes
            Bit depth:          1 byte
            Color type:         1 byte
            Compression method: 1 byte
            Filter method:      1 byte
            Interlace method:   1 byte

         Width and height give the image dimensions in pixels.  They are
         4-byte integers.  Zero is an invalid value.  The maximum for
         each is (2^31)-1 in order to accommodate languages that have
         difficulty with unsigned 4-byte values.

         Bit depth is a single-byte integer giving the number of bits
         per sample or per palette index (not per pixel).  Valid values
         are 1, 2, 4, 8, and 16, although not all values are allowed for
         all color types.

         Color type is a single-byte integer that describes the
         interpretation of the image data.  Color type codes represent
         sums of the following values: 1 (palette used), 2 (color used),
         and 4 (alpha channel used).  Valid values are 0, 2, 3, 4, and
         6.

         Bit depth restrictions for each color type are imposed to
         simplify implementations and to prohibit combinations that do
         not compress well.  Decoders must support all valid
         combinations of bit depth and color type.  The allowed
         combinations are:

            Color    Allowed    Interpretation
            Type    Bit Depths

            0       1,2,4,8,16  Each pixel is a grayscale sample.

            2       8,16        Each pixel is an R,G,B triple.

            3       1,2,4,8     Each pixel is a palette index;
                                a PLTE chunk must appear.

            4       8,16        Each pixel is a grayscale sample,
                                followed by an alpha sample.

            6       8,16        Each pixel is an R,G,B triple,
                                followed by an alpha sample.

         The sample depth is the same as the bit depth except in the
         case of color type 3, in which the sample depth is always 8
         bits.

         Compression method is a single-byte integer that indicates the
         method used to compress the image data.  At present, only
         compression method 0 (deflate/inflate compression with a
         sliding window of at most 32768 bytes) is defined.  All
         standard PNG images must be compressed with this scheme.  The
         compression method field is provided for possible future
         expansion or proprietary variants.  Decoders must check this
         byte and report an error if it holds an unrecognized code.  See
         Deflate/Inflate Compression (Chapter 5) for details.

         Filter method is a single-byte integer that indicates the
         preprocessing method applied to the image data before
         compression.  At present, only filter method 0 (adaptive
         filtering with five basic filter types) is defined.  As with
         the compression method field, decoders must check this byte and
         report an error if it holds an unrecognized code.  See Filter
         Algorithms (Chapter 6) for details.

         Interlace method is a single-byte integer that indicates the
         transmission order of the image data.  Two values are currently
         defined: 0 (no interlace) or 1 (Adam7 interlace).  See
         Interlaced data order (Section 2.6) for details.

      4.1.2. PLTE Palette

         The PLTE chunk contains from 1 to 256 palette entries, each a
         three-byte series of the form:

            Red:   1 byte (0 = black, 255 = red)
            Green: 1 byte (0 = black, 255 = green)
            Blue:  1 byte (0 = black, 255 = blue)

         The number of entries is determined from the chunk length.  A
         chunk length not divisible by 3 is an error.

         This chunk must appear for color type 3, and can appear for
         color types 2 and 6; it must not appear for color types 0 and
         4.  If this chunk does appear, it must precede the first IDAT
         chunk.  There must not be more than one PLTE chunk.

         For color type 3 (indexed color), the PLTE chunk is required.
         The first entry in PLTE is referenced by pixel value 0, the
         second by pixel value 1, etc.  The number of palette entries
         must not exceed the range that can be represented in the image
         bit depth (for example, 2^4 = 16 for a bit depth of 4).  It is
         permissible to have fewer entries than the bit depth would
         allow.  In that case, any out-of-range pixel value found in the
         image data is an error.

         For color types 2 and 6 (truecolor and truecolor with alpha),
         the PLTE chunk is optional.  If present, it provides a
         suggested set of from 1 to 256 colors to which the truecolor
         image can be quantized if the viewer cannot display truecolor
         directly.  If neither PLTE nor sPLT is present, such a viewer
         will need to select colors on its own, but it is often
         preferable for this to be done once by the encoder.  (See
         Recommendations for Encoders: Suggested palettes, Section 9.5.)

         Note that the palette uses 8 bits (1 byte) per sample
         regardless of the image bit depth specification.  In
         particular, the palette is 8 bits deep even when it is a
         suggested quantization of a 16-bit truecolor image.

         There is no requirement that the palette entries all be used by
         the image, nor that they all be different.

      4.1.3. IDAT Image data

         The IDAT chunk contains the actual image data.  To create this
         data:

            1. Begin with image scanlines represented as described in
               Image layout (Section 2.3); the layout and total size of
               this raw data are determined by the fields of IHDR.

            2. Filter the image data according to the filtering method
               specified by the IHDR chunk.  (Note that with filter
               method 0, the only one currently defined, this implies
               prepending a filter-type byte to each scanline.)

            3. Compress the filtered data using the compression method
               specified by the IHDR chunk.

         The IDAT chunk contains the output datastream of the
         compression algorithm.

         To read the image data, reverse this process.

         There can be multiple IDAT chunks; if so, they must appear
         consecutively with no other intervening chunks.  The compressed
         datastream is then the concatenation of the contents of all the
         IDAT chunks.  The encoder can divide the compressed datastream
         into IDAT chunks however it wishes.  (Multiple IDAT chunks are
         allowed so that encoders can work in a fixed amount of memory;
         typically the chunk size will correspond to the encoder's
         buffer size.)  It is important to emphasize that IDAT chunk
         boundaries have no semantic significance and can occur at any
         point in the compressed datastream.  A PNG file in which each
         IDAT chunk contains only one data byte is valid, though
         remarkably wasteful of space.  (For that matter, zero-length
         IDAT chunks are valid, though even more wasteful.)

         See Filter Algorithms (Chapter 6) and Deflate/Inflate
         Compression (Chapter 5) for details.

      4.1.4. IEND Image trailer

         The IEND chunk must appear LAST.  It marks the end of the PNG
         datastream.  The chunk's data field is empty.

   4.2. Ancillary chunks

      All ancillary chunks are optional, in the sense that encoders need
      not write them and decoders can ignore them.  However, encoders
      are encouraged to write the standard ancillary chunks when the
      information is available, and decoders are encouraged to interpret
      these chunks when appropriate and feasible.

      The standard ancillary chunks are listed in alphabetical order.
      This is not necessarily the order in which they would appear in a
      file.

      4.2.1. bKGD Background color

         The bKGD chunk specifies a default background color to present
         the image against.  Note that viewers are not bound to honor
         this chunk; a viewer can choose to use a different background.

         For color type 3 (indexed color), the bKGD chunk contains:

            Palette index:  1 byte

         The value is the palette index of the color to be used as
         background.

         For color types 0 and 4 (grayscale, with or without alpha),
         bKGD contains:

            Gray:  2 bytes, range 0 .. (2^bitdepth)-1

         (If the image bit depth is less than 16, the least significant
         bits are used and the others are 0.)  The value is the gray
         level to be used as background.

         For color types 2 and 6 (truecolor, with or without alpha),
         bKGD contains:

            Red:   2 bytes, range 0 .. (2^bitdepth)-1
            Green: 2 bytes, range 0 .. (2^bitdepth)-1
            Blue:  2 bytes, range 0 .. (2^bitdepth)-1

         (If the image bit depth is less than 16, the least significant
         bits are used and the others are 0.)  This is the RGB color to
         be used as background.

         When present, the bKGD chunk must precede the first IDAT chunk,
         and must follow the PLTE chunk, if any.

         See Recommendations for Decoders: Background color (Section
         10.7).

      4.2.2. cHRM Primary chromaticities and white point

         Applications that need device-independent specification of
         colors in a PNG file can use the cHRM chunk to specify the 1931
         CIE x,y chromaticities of the red, green, and blue primaries
         used in the image, and the referenced white point.  See Color
         Tutorial (Chapter 14) for more information.

         The cHRM chunk contains:

            White Point x: 4 bytes
            White Point y: 4 bytes
            Red x:         4 bytes
            Red y:         4 bytes
            Green x:       4 bytes
            Green y:       4 bytes
            Blue x:        4 bytes
            Blue y:        4 bytes

         Each value is encoded as a 4-byte unsigned integer,
         representing the x or y value times 100000.  For example, a
         value of 0.3127 would be stored as the integer 31270.

         cHRM is allowed in all PNG files, although it is of little
         value for grayscale images.

         If the encoder does not know the chromaticity values, it should
         not write a cHRM chunk; the absence of a cHRM chunk indicates
         that the image's primary colors are device-dependent.

         If the cHRM chunk appears, it must precede the first IDAT
         chunk, and it must also precede the PLTE chunk if present.

         An sRGB chunk or iCCP chunk, when present and recognized,
         overrides the cHRM chunk.

         See the sRGB chunk specification (Paragraph 4.2.9), the iCCP
         chunk specification (Paragraph 4.2.5), Recommendations for
         Encoders: Encoder color handling (Section 9.3), and
         Recommendations for Decoders: Decoder color handling (Section
         10.6).

      4.2.3. gAMA Image gamma

         The gAMA chunk specifies the relationship between the image
         samples and the desired display output intensity as a power
         function:

            sample = light_out ^ gamma

         Here sample and light_out are normalized to the range 0.0
         (minimum intensity) to 1.0 (maximum intensity).  Therefore:

            sample = integer_sample / (2^bitdepth - 1)

         The gAMA chunk contains:

            Gamma: 4 bytes

         The value is encoded as a 4-byte unsigned integer, representing
         gamma times 100000.  For example, a gamma of 1/2.2 would be
         stored as 45455.

         The gamma value has no effect on alpha samples, which are
         always a linear fraction of full opacity.

         If the encoder does not know the image's gamma value, it should
         not write a gAMA chunk; the absence of a gAMA chunk indicates
         that the gamma is unknown.

         Technically, "desired display output intensity" is not specific
         enough; one needs to specify the viewing conditions under which
         the output is desired.  For gAMA these are the reference
         viewing conditions of the sRGB specification [sRGB], which are
         based on ISO standards [ISO-3664]. Adjusting for different
         viewing conditions is a complex process normally handled by a
         Color Management System (CMS).  If this adjustment is not
         performed, the error is usually small.  Applications desiring
         high color fidelity may wish to use an sRGB chunk (see the sRGB
         chunk specification, Paragraph 4.2.9) or an iCCP chunk (see the
         iCCP chunk specification, Paragraph 4.2.5).

         If the gAMA chunk appears, it must precede the first IDAT
         chunk, and it must also precede the PLTE chunk if present.  An
         sRGB chunk or iCCP chunk, when present and recognized,
         overrides the gAMA chunk.

         See Gamma correction (Section 2.7), Recommendations for
         Encoders: Encoder gamma handling (Section 9.2), and
         Recommendations for Decoders: Decoder gamma handling (Section
         10.5).

      4.2.4. hIST Palette histogram

         The hIST chunk gives the approximate usage frequency of each
         color in the color palette.  A hIST chunk can appear only when
         a PLTE chunk appears.  If a viewer is unable to provide all the
         colors listed in the palette, the histogram may help it decide
         how to choose a subset of the colors for display.

         The hIST chunk contains a series of 2-byte (16 bit) unsigned
         integers.  There must be exactly one entry for each entry in
         the PLTE chunk.  Each entry is proportional to the fraction of
         pixels in the image that have that palette index; the exact
         scale factor is chosen by the encoder.

         Histogram entries are approximate, with the exception that a
         zero entry specifies that the corresponding palette entry is
         not used at all in the image.  It is required that a histogram
         entry be nonzero if there are any pixels of that color.

         When the palette is a suggested quantization of a truecolor
         image, the histogram is necessarily approximate, since a
         decoder may map pixels to palette entries differently than the
         encoder did.  In this situation, zero entries should not
         appear.

         The hIST chunk, if it appears, must follow the PLTE chunk, and
         must precede the first IDAT chunk.

         See Rationale: Palette histograms (Section 12.14) and
         Recommendations for Decoders: Suggested-palette and histogram
         usage (Section 10.10).

      4.2.5. iCCP Embedded ICC profile

         If the iCCP chunk is present, the image samples conform to the
         color space represented by the embedded ICC profile as defined
         by the International Color Consortium [ICC]. The color space of
         the ICC profile must be an RGB color space for color images
         (PNG color types 2, 3, and 6), or a monochrome grayscale color
         space for grayscale images (PNG color types 0 and 4).

         The iCCP chunk contains:

            Profile name:       1-79 bytes (character string)
            Null separator:     1 byte
            Compression method: 1 byte
            Compressed profile: n bytes

         The format is like the zTXt chunk.  The profile name can be any
         convenient name for referring to the profile.  It is case-
         sensitive and subject to the same restrictions as a tEXt
         keyword:  it must contain only printable Latin-1
         [ISO/IEC-8859-1] characters (33-126 and 161-255) and spaces
         (32), but no leading, trailing, or consecutive spaces.  The
         only value presently defined for the compression method byte is
         0, meaning zlib datastream with deflate compression (see
         Deflate/Inflate Compression, Chapter 5). Decompression of the
         remainder of the chunk yields the ICC profile.

         An application that writes the iCCP chunk should also write
         gAMA and cHRM chunks that approximate the ICC profile's
         transfer function, for compatibility with applications that do
         not use the iCCP chunk.

         When the iCCP chunk is present, applications that recognize it
         and are capable of color management [ICC] should ignore the
         gAMA and cHRM chunks and use the iCCP chunk instead, but
         applications incapable of full-fledged color management should
         use the gAMA and cHRM chunks if present.

         A file should contain at most one embedded profile, whether
         explicit like iCCP or implicit like sRGB.

         If the iCCP chunk appears, it must precede the first IDAT
         chunk, and it must also precede the PLTE chunk if present.

      4.2.6. pHYs Physical pixel dimensions

         The pHYs chunk specifies the intended pixel size or aspect
         ratio for display of the image.  It contains:

            Pixels per unit, X axis: 4 bytes (unsigned integer)
            Pixels per unit, Y axis: 4 bytes (unsigned integer)
            Unit specifier:          1 byte

         The following values are defined for the unit specifier:

            0: unit is unknown
            1: unit is the meter

         When the unit specifier is 0, the pHYs chunk defines pixel
         aspect ratio only; the actual size of the pixels remains
         unspecified.

         Conversion note: one inch is equal to exactly 0.0254 meters.

         If this ancillary chunk is not present, pixels are assumed to
         be square, and the physical size of each pixel is unknown.

         If present, this chunk must precede the first IDAT chunk.

         See Recommendations for Decoders: Pixel dimensions (Section
         10.2).

      4.2.7. sBIT Significant bits

         To simplify decoders, PNG specifies that only certain sample
         depths can be used, and further specifies that sample values
         should be scaled to the full range of possible values at the
         sample depth.  However, the sBIT chunk is provided in order to
         store the original number of significant bits.  This allows
         decoders to recover the original data losslessly even if the
         data had a sample depth not directly supported by PNG.  We
         recommend that an encoder emit an sBIT chunk if it has
         converted the data from a lower sample depth.

         For color type 0 (grayscale), the sBIT chunk contains a single
         byte, indicating the number of bits that were significant in
         the source data.

         For color type 2 (truecolor), the sBIT chunk contains three
         bytes, indicating the number of bits that were significant in
         the source data for the red, green, and blue channels,
         respectively.

         For color type 3 (indexed color), the sBIT chunk contains three
         bytes, indicating the number of bits that were significant in
         the source data for the red, green, and blue components of the
         palette entries, respectively.

         For color type 4 (grayscale with alpha channel), the sBIT chunk
         contains two bytes, indicating the number of bits that were
         significant in the source grayscale data and the source alpha
         data, respectively.

         For color type 6 (truecolor with alpha channel), the sBIT chunk
         contains four bytes, indicating the number of bits that were
         significant in the source data for the red, green, blue, and
         alpha channels, respectively.

         Each depth specified in sBIT must be greater than zero and less
         than or equal to the sample depth (which is 8 for indexed-color
         images, and the bit depth given in IHDR for other color types).

         A decoder need not pay attention to sBIT: the stored image is a
         valid PNG file of the sample depth indicated by IHDR.  However,
         if the decoder wishes to recover the original data at its
         original precision, this can be done by right-shifting the
         stored samples (the stored palette entries, for an indexed-
         color image).  The encoder must scale the data in such a way
         that the high-order bits match the original data.

         If the sBIT chunk appears, it must precede the first IDAT
         chunk, and it must also precede the PLTE chunk if present.

         See Recommendations for Encoders: Sample depth scaling (Section
         9.1) and Recommendations for Decoders: Sample depth rescaling
         (Section 10.4).

      4.2.8. sPLT Suggested palette

         This chunk can be used to suggest a reduced palette to be used
         when the display device is not capable of displaying the full
         range of colors present in the image.  If present, it provides
         a recommended set of colors, with alpha and frequency
         information, that can be used to construct a reduced palette to
         which the PNG image can be quantized.

         This chunk contains a null-terminated text string that names
         the palette and a one-byte sample depth, followed by a series
         of palette entries, each a six-byte or ten-byte series
         containing five unsigned integers:

            Palette name:    1-79 bytes (character string)
            Null terminator: 1 byte
            Sample depth:    1 byte
            Red:             1 or 2 bytes
            Green:           1 or 2 bytes
            Blue:            1 or 2 bytes
            Alpha:           1 or 2 bytes
            Frequency:       2 bytes
            ...etc...

         There can be any number of entries; a decoder determines the
         number of entries from the remaining chunk length after the
         sample depth byte.  It is an error if this remaining length is
         not divisible by 6 (if the sPLT sample depth is 8) or by 10 (if
         the sPLT sample depth is 16).  Entries must appear in
         decreasing order of frequency.  There is no requirement that
         the entries all be used by the image, nor that they all be
         different.

         The palette name can be any convenient name for referring to
         the palette (for example, "256 color including Macintosh
         default", "256 color including Windows-3.1 default", "Optimal
         512").  It may help applications or people to choose the
         appropriate suggested palette when more than one appears in a
         PNG file.  The palette name is case-sensitive and subject to
         the same restrictions as a tEXt keyword:  it must contain only
         printable Latin-1 [ISO/IEC-8859-1] characters (33-126 and
         161-255) and spaces (32), but no leading, trailing, or
         consecutive spaces.

         The sPLT sample depth must be 8 or 16.

         The red, green, blue, and alpha samples are either one or two
         bytes each, depending on the sPLT sample depth, regardless of
         the image bit depth.  The color samples are not premultiplied
         by alpha, nor are they precomposited against any background.
         An alpha value of 0 means fully transparent, while an alpha
         value of 255 (when the sPLT sample depth is 8) or 65535 (when
         the sPLT sample depth is 16) means fully opaque.  The palette
         samples have the same gamma and chromaticity values as those of
         the PNG image.

         Each frequency value is proportional to the fraction of pixels
         in the image that are closest to that palette entry in RGBA
         space, before the image has been composited against any
         background.  The exact scale factor is chosen by the encoder,
         but should be chosen so that the range of individual values
         reasonably fills the range 0 to 65535.  It is acceptable to
         artificially inflate the frequencies for "important" colors
         such as those in a company logo or in the facial features of a
         portrait.  Zero is a valid frequency meaning the color is
         "least important" or that it is rarely if ever used.  But when
         all of the frequencies are zero, they are meaningless (nothing
         may be inferred about the actual frequencies of the colors).

         The sPLT chunk can appear for any PNG color type.  Note that
         entries in sPLT can fall outside the colorspace of the PNG
         image; for example, in a grayscale PNG, sPLT entries would
         typically satisfy R=G=B, but this is not required.  Similarly,
         sPLT entries can have nonopaque alpha values even when the PNG
         image does not use transparency.

         If sPLT appears, it must precede the first IDAT chunk.  There
         can be multiple sPLT chunks, but if so they must have different
         palette names.

         See Recommendations for Encoders: Suggested palettes (Section
         9.5) and Recommendations for Decoders: Suggested-palette and
         histogram usage (Section 10.10)

      4.2.9. sRGB Standard RGB color space

         If the sRGB chunk is present, the image samples conform to the
         sRGB color space [sRGB], and should be displayed using the
         specified rendering intent as defined by the International
         Color Consortium [ICC].

         The sRGB chunk contains:

            Rendering intent: 1 byte

         The following values are defined for the rendering intent:

            0: Perceptual
            1: Relative colorimetric
            2: Saturation
            3: Absolute colorimetric

         Perceptual intent is for images preferring good adaptation to
         the output device gamut at the expense of colorimetric
         accuracy, like photographs.

         Relative colorimetric intent is for images requiring color
         appearance matching (relative to the output device white
         point), like logos.

         Saturation intent is for images preferring preservation of
         saturation at the expense of hue and lightness, like charts and
         graphs.

         Absolute colorimetric intent is for images requiring
         preservation of absolute colorimetry, like proofs (previews of
         images destined for a different output device).

         An application that writes the sRGB chunk should also write a
         gAMA chunk (and perhaps a cHRM chunk) for compatibility with
         applications that do not use the sRGB chunk.  In this
         situation, only the following values may be used:

            gAMA:
            Gamma:         45455

            cHRM:
            White Point x: 31270
            White Point y: 32900
            Red x:         64000
            Red y:         33000
            Green x:       30000
            Green y:       60000
            Blue x:        15000
            Blue y:         6000

         When the sRGB chunk is present, applications that recognize it
         and are capable of color management [ICC] must ignore the gAMA
         and cHRM chunks and use the sRGB chunk instead.

         Applications that recognize the sRGB chunk but are not capable
         of full-fledged color management must also ignore the gAMA and
         cHRM chunks, because the applications already know what values
         those chunks should contain.  The applications must therefore
         use the values of gAMA and cHRM given above as if they had
         appeared in gAMA and cHRM chunks.

         If the sRGB chunk appears, it must precede the first IDAT
         chunk, and it must also precede the PLTE chunk if present.  The
         sRGB and iCCP chunks should not both appear.

      4.2.10. tEXt Textual data

         Textual information that the encoder wishes to record with the
         image can be stored in tEXt chunks.  Each tEXt chunk contains a
         keyword and a text string, in the format:

            Keyword:        1-79 bytes (character string)
            Null separator: 1 byte
            Text:           n bytes (character string)

         The keyword and text string are separated by a zero byte (null
         character).  Neither the keyword nor the text string can
         contain a null character.  Note that the text string is not
         null-terminated (the length of the chunk is sufficient
         information to locate the ending).  The keyword must be at
         least one character and less than 80 characters long.  The text
         string can be of any length from zero bytes up to the maximum
         permissible chunk size less the length of the keyword and
         separator.

         Any number of tEXt chunks can appear, and more than one with
         the same keyword is permissible.

         The keyword indicates the type of information represented by
         the text string.  The following keywords are predefined and
         should be used where appropriate:

            Title            Short (one line) title or caption for image
            Author           Name of image's creator
            Description      Description of image (possibly long)
            Copyright        Copyright notice
            Creation Time    Time of original image creation
            Software         Software used to create the image
            Disclaimer       Legal disclaimer
            Warning          Warning of nature of content
            Source           Device used to create the image
            Comment          Miscellaneous comment; conversion from
                             GIF comment

         For the Creation Time keyword, the date format defined in
         section 5.2.14 of RFC 1123 is suggested, but not required
         [RFC-1123]. Decoders should allow for free-format text
         associated with this or any other keyword.

         Other keywords may be invented for other purposes.  Keywords of
         general interest can be registered with the maintainers of the
         PNG specification.  However, it is also permitted to use
         private unregistered keywords.  (Private keywords should be
         reasonably self-explanatory, in order to minimize the chance
         that the same keyword will be used for incompatible purposes by
         different people.)

         Both keyword and text are interpreted according to the ISO/IEC
         8859-1 (Latin-1) character set [ISO/IEC-8859-1]. The text
         string can contain any Latin-1 character.  Newlines in the text
         string should be represented by a single linefeed character
         (decimal 10); use of other control characters in the text is
         discouraged.

         Keywords must contain only printable Latin-1 characters and
         spaces; that is, only character codes 32-126 and 161-255
         decimal are allowed.  To reduce the chances for human
         misreading of a keyword, leading and trailing spaces are
         forbidden, as are consecutive spaces.  Note also that the non-
         breaking space (code 160) is not permitted in keywords, since
         it is visually indistinguishable from an ordinary space.

         Keywords must be spelled exactly as registered, so that
         decoders can use simple literal comparisons when looking for
         particular keywords.  In particular, keywords are considered
         case-sensitive.

         See Recommendations for Encoders: Text chunk processing
         (Section 9.7) and Recommendations for Decoders: Text chunk
         processing (Section 10.11).

      4.2.11. tIME Image last-modification time

         The tIME chunk gives the time of the last image modification
         (not the time of initial image creation).  It contains:

            Year:   2 bytes (complete; for example, 1995, not 95)
            Month:  1 byte (1-12)
            Day:    1 byte (1-31)
            Hour:   1 byte (0-23)
            Minute: 1 byte (0-59)
            Second: 1 byte (0-60)    (yes, 60, for leap seconds; not 61,
                                      a common error)

         Universal Time (UTC, also called GMT) should be specified
         rather than local time.

         The tIME chunk is intended for use as an automatically-applied
         time stamp that is updated whenever the image data is changed.
         It is recommended that tIME not be changed by PNG editors that
         do not change the image data.  See also the Creation Time tEXt
         keyword, which can be used for a user-supplied time.

      4.2.12. tRNS Transparency

         The tRNS chunk specifies that the image uses simple
         transparency: either alpha values associated with palette
         entries (for indexed-color images) or a single transparent
         color (for grayscale and truecolor images).  Although simple
         transparency is not as elegant as the full alpha channel, it
         requires less storage space and is sufficient for many common
         cases.

         For color type 3 (indexed color), the tRNS chunk contains a
         series of one-byte alpha values, corresponding to entries in
         the PLTE chunk:

            Alpha for palette index 0:  1 byte
            Alpha for palette index 1:  1 byte
            ...etc...

         Each entry indicates that pixels of the corresponding palette
         index must be treated as having the specified alpha value.
         Alpha values have the same interpretation as in an 8-bit full
         alpha channel: 0 is fully transparent, 255 is fully opaque,
         regardless of image bit depth.  The tRNS chunk must not contain
         more alpha values than there are palette entries, but tRNS can
         contain fewer values than there are palette entries.  In this
         case, the alpha value for all remaining palette entries is
         assumed to be 255.  In the common case in which only palette
         index 0 need be made transparent, only a one-byte tRNS chunk is
         needed.

         For color type 0 (grayscale), the tRNS chunk contains a single
         gray level value, stored in the format:

            Gray:  2 bytes, range 0 .. (2^bitdepth)-1

         (If the image bit depth is less than 16, the least significant
         bits are used and the others are 0.)  Pixels of the specified
         gray level are to be treated as transparent (equivalent to
         alpha value 0); all other pixels are to be treated as fully
         opaque (alpha value (2^bitdepth)-1).

         For color type 2 (truecolor), the tRNS chunk contains a single
         RGB color value, stored in the format:

            Red:   2 bytes, range 0 .. (2^bitdepth)-1
            Green: 2 bytes, range 0 .. (2^bitdepth)-1
            Blue:  2 bytes, range 0 .. (2^bitdepth)-1

         (If the image bit depth is less than 16, the least significant
         bits are used and the others are 0.)  Pixels of the specified
         color value are to be treated as transparent (equivalent to
         alpha value 0); all other pixels are to be treated as fully
         opaque (alpha value (2^bitdepth)-1).

         tRNS is prohibited for color types 4 and 6, since a full alpha
         channel is already present in those cases.

         Note: when dealing with 16-bit grayscale or truecolor data, it
         is important to compare both bytes of the sample values to
         determine whether a pixel is transparent.  Although decoders
         may drop the low-order byte of the samples for display, this
         must not occur until after the data has been tested for
         transparency.  For example, if the grayscale level 0x0001 is
         specified to be transparent, it would be incorrect to compare
         only the high-order byte and decide that 0x0002 is also
         transparent.

         When present, the tRNS chunk must precede the first IDAT chunk,
         and must follow the PLTE chunk, if any.

      4.2.13. zTXt Compressed textual data

         The zTXt chunk contains textual data, just as tEXt does;
         however, zTXt takes advantage of compression.  The zTXt and
         tEXt chunks are semantically equivalent, but zTXt is
         recommended for storing large blocks of text.

         A zTXt chunk contains:

            Keyword:            1-79 bytes (character string)
            Null separator:     1 byte
            Compression method: 1 byte
            Compressed text:    n bytes

         The keyword and null separator are exactly the same as in the
         tEXt chunk.  Note that the keyword is not compressed.  The
         compression method byte identifies the compression method used
         in this zTXt chunk.  The only value presently defined for it is
         0 (deflate/inflate compression).  The compression method byte
         is followed by a compressed datastream that makes up the
         remainder of the chunk.  For compression method 0, this
         datastream adheres to the zlib datastream format (see
         Deflate/Inflate Compression, Chapter 5).  Decompression of this
         datastream yields Latin-1 text that is identical to the text
         that would be stored in an equivalent tEXt chunk.

         Any number of zTXt and tEXt chunks can appear in the same file.
         See the preceding definition of the tEXt chunk for the
         predefined keywords and the recommended format of the text.

         See Recommendations for Encoders: Text chunk processing
         (Section 9.7) and Recommendations for Decoders: Text chunk
         processing (Section 10.11).

   4.3. Summary of standard chunks

      This table summarizes some properties of the standard chunk types.

         Critical chunks (must appear in this order, except PLTE
                          is optional):

                 Name  Multiple  Ordering constraints
                         OK?

                 IHDR    No      Must be first
                 PLTE    No      Before IDAT
                 IDAT    Yes     Multiple IDATs must be consecutive
                 IEND    No      Must be last

         Ancillary chunks (need not appear in this order):

                 Name  Multiple  Ordering constraints
                         OK?

                 cHRM    No      Before PLTE and IDAT
                 gAMA    No      Before PLTE and IDAT
                 iCCP    No      Before PLTE and IDAT
                 sBIT    No      Before PLTE and IDAT
                 sRGB    No      Before PLTE and IDAT
                 bKGD    No      After PLTE; before IDAT
                 hIST    No      After PLTE; before IDAT
                 tRNS    No      After PLTE; before IDAT
                 pHYs    No      Before IDAT
                 sPLT    Yes     Before IDAT
                 tIME    No      None
                 tEXt    Yes     None
                 zTXt    Yes     None

      Standard keywords for tEXt and zTXt chunks:

         Title            Short (one line) title or caption for image
         Author           Name of image's creator
         Description      Description of image (possibly long)
         Copyright        Copyright notice
         Creation Time    Time of original image creation
         Software         Software used to create the image
         Disclaimer       Legal disclaimer
         Warning          Warning of nature of content
         Source           Device used to create the image
         Comment          Miscellaneous comment; conversion from
                          GIF comment

   4.4. Additional chunk types

      Additional public PNG chunk types are defined in the document
      "Extensions to the PNG-1.1 Specification, Version 1.1.0"
      [PNG-EXTENSIONS].  Chunks described there are expected to be less
      widely supported than those defined in this specification.
      However, application authors are encouraged to use those chunk
      types whenever appropriate for their applications.  Additional
      chunk types can be proposed for inclusion in that list by
      contacting the PNG specification maintainers at
      [e-mail] or at [e-mail].

      New public chunks will be registered only if they are of use to
      others and do not violate the design philosophy of PNG.  Chunk
      registration is not automatic, although it is the intent of the
      authors that it be straightforward when a new chunk of potentially
      wide application is needed.  Note that the creation of new
      critical chunk types is discouraged unless absolutely necessary.

      Applications can also use private chunk types to carry data that
      is not of interest to other applications.  See Recommendations for
      Encoders: Use of private chunks (Section 9.8).

      Decoders must be prepared to encounter unrecognized public or
      private chunk type codes.  Unrecognized chunk types must be
      handled as described in Chunk naming conventions (Section 3.3).

5. Deflate/Inflate Compression

   PNG compression method 0 (the only compression method presently
   defined for PNG) specifies deflate/inflate compression with a sliding
   window of at most 32768 bytes.  Deflate compression is an LZ77
   derivative used in zip, gzip, pkzip, and related programs.  Extensive
   research has been done supporting its patent-free status.  Portable C
   implementations are freely available.

   Deflate-compressed datastreams within PNG are stored in the "zlib"
   format, which has the structure:

      Compression method/flags code: 1 byte
      Additional flags/check bits:   1 byte
      Compressed data blocks:        n bytes
      Check value:                   4 bytes

   Further details on this format are given in the zlib specification
   [RFC-1950].

   For PNG compression method 0, the zlib compression method/flags code
   must specify method code 8 ("deflate" compression) and an LZ77 window
   size of not more than 32768 bytes.  Note that the zlib compression
   method number is not the same as the PNG compression method number.
   The additional flags must not specify a preset dictionary.  A PNG
   decoder must be able to decompress any valid zlib datastream that
   satisfies these additional constraints.

   If the data to be compressed contains 16384 bytes or fewer, the
   encoder can set the window size by rounding up to a power of 2 (256
   minimum).  This decreases the memory required not only for encoding
   but also for decoding, without adversely affecting the compression
   ratio.

   The compressed data within the zlib datastream is stored as a series
   of blocks, each of which can represent raw (uncompressed) data,
   LZ77-compressed data encoded with fixed Huffman codes, or LZ77-
   compressed data encoded with custom Huffman codes.  A marker bit in
   the final block identifies it as the last block, allowing the decoder
   to recognize the end of the compressed datastream.  Further details
   on the compression algorithm and the encoding are given in the
   deflate specification [RFC-1951].

   The check value stored at the end of the zlib datastream is
   calculated on the uncompressed data represented by the datastream.
   Note that the algorithm used is not the same as the CRC calculation
   used for PNG chunk check values.  The zlib check value is useful
   mainly as a cross-check that the deflate and inflate algorithms are
   implemented correctly.  Verifying the chunk CRCs provides adequate
   confidence that the PNG file has been transmitted undamaged.

   In a PNG file, the concatenation of the contents of all the IDAT
   chunks makes up a zlib datastream as specified above.  This
   datastream decompresses to filtered image data as described elsewhere
   in this document.

   It is important to emphasize that the boundaries between IDAT chunks
   are arbitrary and can fall anywhere in the zlib datastream.  There is
   not necessarily any correlation between IDAT chunk boundaries and
   deflate block boundaries or any other feature of the zlib data.  For
   example, it is entirely possible for the terminating zlib check value
   to be split across IDAT chunks.

   In the same vein, there is no required correlation between the
   structure of the image data (i.e., scanline boundaries) and deflate
   block boundaries or IDAT chunk boundaries.  The complete image data
   is represented by a single zlib datastream that is stored in some
   number of IDAT chunks; a decoder that assumes any more than this is
   incorrect.  (Of course, some encoder implementations may emit files
   in which some of these structures are indeed related.  But decoders
   cannot rely on this.)

   PNG also uses zlib datastreams in zTXt and iCCP chunks, where the
   remainder of the chunk following the compression method byte is a
   zlib datastream as specified above.  Unlike the image data, such
   datastreams are not split across chunks; each zTXt or iCCP chunk
   contains an independent zlib datastream.

   Additional documentation and portable C code for deflate and inflate
   are available from the Info-ZIP archives at
   ftp://ftp.cdrom.com/pub/infozip/.

6. Filter Algorithms

   This chapter describes the filter algorithms that can be applied
   before compression.  The purpose of these filters is to prepare the
   image data for optimum compression.

   6.1. Filter types

      PNG filter method 0 defines five basic filter types:

         Type    Name

         0       None
         1       Sub
         2       Up
         3       Average
         4       Paeth

      (Note that filter method 0 in IHDR specifies exactly this set of
      five filter types.  If the set of filter types is ever extended, a
      different filter method number will be assigned to the extended
      set, so that decoders need not decompress the data to discover
      that it contains unsupported filter types.)

      The encoder can choose which of these filter algorithms to apply
      on a scanline-by-scanline basis.  In the image data sent to the
      compression step, each scanline is preceded by a filter-type byte
      that specifies the filter algorithm used for that scanline.

      Filtering algorithms are applied to bytes, not to pixels,
      regardless of the bit depth or color type of the image.  The
      filtering algorithms work on the byte sequence formed by a
      scanline that has been represented as described in Image layout
      (Section 2.3). If the image includes an alpha channel, the alpha
      data is filtered in the same way as the image data.

      When the image is interlaced, each pass of the interlace pattern
      is treated as an independent image for filtering purposes.  The
      filters work on the byte sequences formed by the pixels actually
      transmitted during a pass, and the "previous scanline" is the one
      previously transmitted in the same pass, not the one adjacent in
      the complete image.  Note that the subimage transmitted in any one
      pass is always rectangular, but is of smaller width and/or height
      than the complete image.  Filtering is not applied when this
      subimage is empty.

      For all filters, the bytes "to the left of" the first pixel in a
      scanline must be treated as being zero.  For filters that refer to
      the prior scanline, the entire prior scanline must be treated as
      being zeroes for the first scanline of an image (or of a pass of
      an interlaced image).

      To reverse the effect of a filter, the decoder must use the
      decoded values of the prior pixel on the same line, the pixel
      immediately above the current pixel on the prior line, and the
      pixel just to the left of the pixel above.  This implies that at
      least one scanline's worth of image data will have to be stored by
      the decoder at all times.  Even though some filter types do not
      refer to the prior scanline, the decoder will always need to store
      each scanline as it is decoded, since the next scanline might use
      a filter that refers to it.

      PNG imposes no restriction on which filter types can be applied to
      an image.  However, the filters are not equally effective on all
      types of data.  See Recommendations for Encoders: Filter selection
      (Section 9.6).

      See also Rationale: Filtering (Section 12.9).

   6.2. Filter type 0: None

      With the None() filter, the scanline is transmitted unmodified; it
      is necessary only to insert a filter-type byte before the data.

   6.3. Filter type 1: Sub

      The Sub() filter transmits the difference between each byte and
      the value of the corresponding byte of the prior pixel.

      To compute the Sub() filter, apply the following formula to each
      byte of the scanline:

         Sub(x) = Raw(x) - Raw(x-bpp)

      where x ranges from zero to the number of bytes representing the
      scanline minus one, Raw() refers to the raw data byte at that byte
      position in the scanline, and bpp is defined as the number of
      bytes per complete pixel, rounding up to one.  For example, for
      color type 2 with a bit depth of 16, bpp is equal to 6 (three
      samples, two bytes per sample); for color type 0 with a bit depth
      of 2, bpp is equal to 1 (rounding up); for color type 4 with a bit
      depth of 16, bpp is equal to 4 (two-byte grayscale sample, plus
      two-byte alpha sample).

      Note this computation is done for each byte, regardless of bit
      depth.  In a 16-bit image, each MSB is predicted from the
      preceding MSB and each LSB from the preceding LSB, because of the
      way that bpp is defined.

      Unsigned arithmetic modulo 256 is used, so that both the inputs
      and outputs fit into bytes.  The sequence of Sub values is
      transmitted as the filtered scanline.

      For all x < 0, assume Raw(x) = 0.

      To reverse the effect of the Sub() filter after decompression,
      output the following value:

         Sub(x) + Raw(x-bpp)

      (computed mod 256), where Raw() refers to the bytes already
      decoded.

   6.4. Filter type 2: Up

      The Up() filter is just like the Sub() filter except that the
      pixel immediately above the current pixel, rather than just to its
      left, is used as the predictor.

      To compute the Up() filter, apply the following formula to each
      byte of the scanline:

         Up(x) = Raw(x) - Prior(x)

      where x ranges from zero to the number of bytes representing the
      scanline minus one, Raw() refers to the raw data byte at that byte
      position in the scanline, and Prior(x) refers to the unfiltered
      bytes of the prior scanline.

      Note this is done for each byte, regardless of bit depth.
      Unsigned arithmetic modulo 256 is used, so that both the inputs
      and outputs fit into bytes.  The sequence of Up values is
      transmitted as the filtered scanline.

      On the first scanline of an image (or of a pass of an interlaced
      image), assume Prior(x) = 0 for all x.

      To reverse the effect of the Up() filter after decompression,
      output the following value:

         Up(x) + Prior(x)

      (computed mod 256), where Prior() refers to the decoded bytes of
      the prior scanline.

   6.5. Filter type 3: Average

      The Average() filter uses the average of the two neighboring
      pixels (left and above) to predict the value of a pixel.

      To compute the Average() filter, apply the following formula to
      each byte of the scanline:

         Average(x) = Raw(x) - floor((Raw(x-bpp)+Prior(x))/2)

      where x ranges from zero to the number of bytes representing the
      scanline minus one, Raw() refers to the raw data byte at that byte
      position in the scanline, Prior() refers to the unfiltered bytes
      of the prior scanline, and bpp is defined as for the Sub() filter.

      Note this is done for each byte, regardless of bit depth.  The
      sequence of Average values is transmitted as the filtered
      scanline.

      The subtraction of the predicted value from the raw byte must be
      done modulo 256, so that both the inputs and outputs fit into
      bytes.  However, the sum Raw(x-bpp)+Prior(x) must be formed
      without overflow (using at least nine-bit arithmetic).  floor()
      indicates that the result of the division is rounded to the next
      lower integer if fractional; in other words, it is an integer
      division or right shift operation.

      For all x < 0, assume Raw(x) = 0.  On the first scanline of an
      image (or of a pass of an interlaced image), assume Prior(x) = 0
      for all x.

      To reverse the effect of the Average() filter after decompression,
      output the following value:

         Average(x) + floor((Raw(x-bpp)+Prior(x))/2)

      where the result is computed mod 256, but the prediction is
      calculated in the same way as for encoding.  Raw() refers to the
      bytes already decoded, and Prior() refers to the decoded bytes of
      the prior scanline.

   6.6. Filter type 4: Paeth

      The Paeth() filter computes a simple linear function of the three
      neighboring pixels (left, above, upper left), then chooses as
      predictor the neighboring pixel closest to the computed value.
      This technique is due to Alan W. Paeth [PAETH].

      To compute the Paeth() filter, apply the following formula to each
      byte of the scanline:

         Paeth(x) = Raw(x) -
                    PaethPredictor(Raw(x-bpp), Prior(x), Prior(x-bpp))

      where x ranges from zero to the number of bytes representing the
      scanline minus one, Raw() refers to the raw data byte at that byte
      position in the scanline, Prior() refers to the unfiltered bytes
      of the prior scanline, and bpp is defined as for the Sub() filter.

      Note this is done for each byte, regardless of bit depth.
      Unsigned arithmetic modulo 256 is used, so that both the inputs
      and outputs fit into bytes.  The sequence of Paeth values is
      transmitted as the filtered scanline.

      The PaethPredictor() function is defined by the following
      pseudocode:

         function PaethPredictor (a, b, c)
         begin
              ; a = left, b = above, c = upper left
              p := a + b - c        ; initial estimate
              pa := abs(p - a)      ; distances to a, b, c
              pb := abs(p - b)
              pc := abs(p - c)
              ; return nearest of a,b,c,
              ; breaking ties in order a,b,c.
              if pa <= pb AND pa <= pc then return a
              else if pb <= pc then return b
              else return c
         end

      The calculations within the PaethPredictor() function must be
      performed exactly, without overflow.  Arithmetic modulo 256 is to
      be used only for the final step of subtracting the function result
      from the target byte value.

      Note that the order in which ties are broken is critical and must
      not be altered.  The tie break order is: pixel to the left, pixel
      above, pixel to the upper left.  (This order differs from that
      given in Paeth's article.)

      For all x < 0, assume Raw(x) = 0 and Prior(x) = 0.  On the first
      scanline of an image (or of a pass of an interlaced image), assume
      Prior(x) = 0 for all x.

      To reverse the effect of the Paeth() filter after decompression,
      output the following value:

         Paeth(x) + PaethPredictor(Raw(x-bpp), Prior(x), Prior(x-bpp))

      (computed mod 256), where Raw() and Prior() refer to bytes already
      decoded.  Exactly the same PaethPredictor() function is used by
      both encoder and decoder.

7. Chunk Ordering Rules

   To allow new chunk types to be added to PNG, it is necessary to
   establish rules about the ordering requirements for all chunk types.
   Otherwise a PNG editing program cannot know what to do when it
   encounters an unknown chunk.

   We define a "PNG editor" as a program that modifies a PNG file and
   wishes to preserve as much as possible of the ancillary information
   in the file.  Two examples of PNG editors are a program that adds or
   modifies text chunks, and a program that adds a suggested palette to
   a truecolor PNG file.  Ordinary image editors are not PNG editors in
   this sense, because they usually discard all unrecognized information
   while reading in an image.  (Note: we strongly encourage programs
   handling PNG files to preserve ancillary information whenever
   possible.)

   As an example of possible problems, consider a hypothetical new
   ancillary chunk type that is safe-to-copy and is required to appear
   after PLTE if PLTE is present.  If our program to add a suggested
   PLTE does not recognize this new chunk, it may insert PLTE in the
   wrong place, namely after the new chunk.  We could prevent such
   problems by requiring PNG editors to discard all unknown chunks, but
   that is a very unattractive solution.  Instead, PNG requires
   ancillary chunks not to have ordering restrictions like this.

   To prevent this type of problem while allowing for future extension,
   we put some constraints on both the behavior of PNG editors and the
   allowed ordering requirements for chunks.

   7.1. Behavior of PNG editors

      The rules for PNG editors are:

          * When copying an unknown unsafe-to-copy ancillary chunk, a
            PNG editor must not move the chunk relative to any critical
            chunk.  It can relocate the chunk freely relative to other
            ancillary chunks that occur between the same pair of
            critical chunks.  (This is well defined since the editor
            must not add, delete, modify, or reorder critical chunks if
            it is preserving unknown unsafe-to-copy chunks.)

          * When copying an unknown safe-to-copy ancillary chunk, a PNG
            editor must not move the chunk from before IDAT to after
            IDAT or vice versa.  (This is well defined because IDAT is
            always present.)  Any other reordering is permitted.

          * When copying a known ancillary chunk type, an editor need
            only honor the specific chunk ordering rules that exist for
            that chunk type.  However, it can always choose to apply the
            above general rules instead.

          * PNG editors must give up on encountering an unknown critical
            chunk type, because there is no way to be certain that a
            valid file will result from modifying a file containing such
            a chunk.  (Note that simply discarding the chunk is not good
            enough, because it might have unknown implications for the
            interpretation of other chunks.)

      These rules are expressed in terms of copying chunks from an input
      file to an output file, but they apply in the obvious way if a PNG
      file is modified in place.

      See also Chunk naming conventions (Section 3.3).

   7.2. Ordering of ancillary chunks

      The ordering rules for an ancillary chunk type cannot be any
      stricter than this:

          * Unsafe-to-copy chunks can have ordering requirements
            relative to critical chunks.

          * Safe-to-copy chunks can have ordering requirements relative
            to IDAT.

      The actual ordering rules for any particular ancillary chunk type
      may be weaker.  See for example the ordering rules for the
      standard ancillary chunk types (Summary of standard chunks,
      Section 4.3).

      Decoders must not assume more about the positioning of any
      ancillary chunk than is specified by the chunk ordering rules.  In
      particular, it is never valid to assume that a specific ancillary
      chunk type occurs with any particular positioning relative to
      other ancillary chunks.  (For example, it is unsafe to assume that
      your private ancillary chunk occurs immediately before IEND.  Even
      if your application always writes it there, a PNG editor might
      have inserted some other ancillary chunk after it.  But you can
      safely assume that your chunk will remain somewhere between IDAT
      and IEND.)

   7.3. Ordering of critical chunks

      Critical chunks can have arbitrary ordering requirements, because
      PNG editors are required to give up if they encounter unknown
      critical chunks.  For example, IHDR has the special ordering rule
      that it must always appear first.  A PNG editor, or indeed any
      PNG-writing program, must know and follow the ordering rules for
      any critical chunk type that it can emit.

8. Miscellaneous Topics

   8.1. File name extension

      On systems where file names customarily include an extension
      signifying file type, the extension ".png" is recommended for PNG
      files.  Lowercase ".png" is preferred if file names are case-
      sensitive.

   8.2. Internet media type

      The Internet Assigned Numbers Authority (IANA) has registered
      "image/png" as the Internet Media Type for PNG [RFC-2045],
      [RFC-2048]. For robustness, decoders may choose to also support
      the interim media type "image/x-png" that was in use before
      registration was complete.

   8.3. Macintosh file layout

      In the Apple Macintosh system, the following conventions are
      recommended:

          * The four-byte file type code for PNG files is "PNGf".  (This
            code has been registered with Apple for PNG files.)  The
            creator code will vary depending on the creating
            application.

          * The contents of the data fork must be a PNG file exactly as
            described in the rest of this specification.

          * The contents of the resource fork are unspecified.  It may
            be empty or may contain application-dependent resources.

          * When transferring a Macintosh PNG file to a non-Macintosh
            system, only the data fork should be transferred.

   8.4. Multiple-image extension

      PNG itself is strictly a single-image format.  However, it may be
      necessary to store multiple images within one file; for example,
      this is needed to convert some GIF files.  In the future, a
      multiple-image format based on PNG may be defined.  Such a format
      will be considered a separate file format and will have a
      different signature.  PNG-supporting applications may or may not
      choose to support the multiple-image format.

      See Rationale: Why not these features? (Section 12.3).

   8.5. Security considerations

      A PNG file or datastream is composed of a collection of explicitly
      typed "chunks".  Chunks whose contents are defined by the
      specification could actually contain anything, including malicious
      code.  But there is no known risk that such malicious code could
      be executed on the recipient's computer as a result of decoding
      the PNG image.

      The possible security risks associated with private chunk types
      and future chunk types cannot be specified at this time.  Security
      issues will be considered when evaluating chunks proposed for
      registration as public chunks.  There is no additional security
      risk associated with unknown or unimplemented chunk types, because
      such chunks will be ignored, or at most be copied into another PNG
      file.

      The tEXt and zTXt chunks contain keywords and data that is meant
      to be displayed as plain text.  The iCCP, sPLT, and some public
      "extension" chunks contain keywords meant to be displayed as plain
      text.  It is possible that if a decoder displays such text without
      filtering out control characters, especially the ESC (escape)
      character, certain systems or terminals could behave in
      undesirable and insecure ways.  We recommend that decoders filter
      out control characters to avoid this risk; see Recommendations for
      Decoders:  Text chunk processing (Section 10.11).

      Every chunk begins with a length field, making it easier to write
      decoders invulnerable to fraudulent chunks that attempt to
      overflow buffers.  The CRC at the end of every chunk provides a
      robust defense against accidentally corrupted data.  Also, the PNG
      signature bytes provide early detection of common file
      transmission errors.

      A decoder that fails to check CRCs could be subject to data
      corruption.  The only likely consequence of such corruption is
      incorrectly displayed pixels within the image.  Worse things might
      happen if the CRC of the IHDR chunk is not checked and the width
      or height fields are corrupted.  See Recommendations for Decoders:
      Error checking (Section 10.1).

      A poorly written decoder might be subject to buffer overflow,
      because chunks can be extremely large, up to (2^31)-1 bytes long.
      But properly written decoders will handle large chunks without
      difficulty.

9. Recommendations for Encoders

   This chapter gives some recommendations for encoder behavior.  The
   only absolute requirement on a PNG encoder is that it produce files
   that conform to the format specified in the preceding chapters.
   However, best results will usually be achieved by following these
   recommendations.

   9.1. Sample depth scaling

      When encoding input samples that have a sample depth that cannot
      be directly represented in PNG, the encoder must scale the samples
      up to a sample depth that is allowed by PNG.  The most accurate
      scaling method is the linear equation

         output = ROUND(input * MAXOUTSAMPLE / MAXINSAMPLE)

      where the input samples range from 0 to MAXINSAMPLE and the
      outputs range from 0 to MAXOUTSAMPLE (which is (2^sampledepth)-1).

      A close approximation to the linear scaling method can be achieved
      by "left bit replication", which is shifting the valid bits to
      begin in the most significant bit and repeating the most
      significant bits into the open bits.  This method is often faster
      to compute than linear scaling.  As an example, assume that 5-bit
      samples are being scaled up to 8 bits.  If the source sample value
      is 27 (in the range from 0-31), then the original bits are:

         4 3 2 1 0
         ---------
         1 1 0 1 1

      Left bit replication gives a value of 222:

         7 6 5 4 3  2 1 0
         ----------------
         1 1 0 1 1  1 1 0
         |=======|  |===|
             |      Leftmost Bits Repeated to Fill Open Bits
             |
         Original Bits

      which matches the value computed by the linear equation.  Left bit
      replication usually gives the same value as linear scaling and is
      never off by more than one.

      A distinctly less accurate approximation is obtained by simply
      left-shifting the input value and filling the low order bits with
      zeroes.  This scheme cannot reproduce white exactly, since it does
      not generate an all-ones maximum value; the net effect is to
      darken the image slightly.  This method is not recommended in
      general, but it does have the effect of improving compression,
      particularly when dealing with greater-than-eight-bit sample
      depths.  Since the relative error introduced by zero-fill scaling
      is small at high sample depths, some encoders may choose to use
      it.  Zero-fill must not be used for alpha channel data, however,
      since many decoders will special-case alpha values of all zeroes
      and all ones.  It is important to represent both those values
      exactly in the scaled data.

      When the encoder writes an sBIT chunk, it is required to do the
      scaling in such a way that the high-order bits of the stored
      samples match the original data.  That is, if the sBIT chunk
      specifies a sample depth of S, the high-order S bits of the stored
      data must agree with the original S-bit data values.  This allows
      decoders to recover the original data by shifting right.  The
      added low-order bits are not constrained.  Note that all the above
      scaling methods meet this restriction.

      When scaling up source data, it is recommended that the low-order
      bits be filled consistently for all samples; that is, the same
      source value should generate the same sample value at any pixel
      position.  This improves compression by reducing the number of
      distinct sample values.  However, this is not a requirement, and
      some encoders may choose not to follow it.  For example, an
      encoder might instead dither the low-order bits, improving
      displayed image quality at the price of increasing file size.

      In some applications the original source data may have a range
      that is not a power of 2.  The linear scaling equation still works
      for this case, although the shifting methods do not.  It is
      recommended that an sBIT chunk not be written for such images,
      since sBIT suggests that the original data range was exactly
      0..(2^S)-1.

   9.2. Encoder gamma handling

      See Gamma Tutorial (Chapter 13) if you aren't already familiar
      with gamma issues.

      Encoders capable of full-fledged color management [ICC] will
      perform more sophisticated calculations than those described here,
      and they may choose to use the iCCP chunk.  Encoders that know
      that their image samples conform to the sRGB specification [sRGB]
      should use the sRGB chunk and not perform gamma handling.
      Otherwise, this section applies.

      The encoder has two gamma-related decisions to make.  First, it
      must decide how to transform whatever image samples it has into
      the image samples that will go into the PNG file.  Second, it must
      decide what value to write into the gAMA chunk.

      The rule for the second decision is simply to write whatever value
      will cause a decoder to do what you want.  See Recommendations for
      Decoders: Decoder gamma handling (Section 10.5).

      The first decision depends on the nature of the image samples and
      their precision.  If the samples represent light intensity in
      floating-point or high-precision integer form (perhaps from a
      computer image renderer), then the encoder may perform "gamma
      encoding" (applying a power function with exponent less than 1)
      before quantizing the data to integer values for output to the
      file.  This results in fewer banding artifacts at a given sample
      depth, or allows smaller samples while retaining the same visual
      quality.  An intensity level expressed as a floating-point value
      in the range 0 to 1 can be converted to a file image sample by

         sample = intensity ^ encoding_exponent
         integer_sample = ROUND(sample * (2^bitdepth - 1))

      If the intensity in the equation is the desired display output
      intensity, then the encoding exponent is the gamma value to be
      written to the file, by the definition of gAMA (See the gAMA chunk
      specification, Paragraph 4.2.3). But if the intensity available to
      the encoder is the original scene intensity, another
      transformation may be needed.  Sometimes the displayed image
      should have higher contrast than the original image; in other
      words, the end-to-end transfer function from original scene to
      display output should have an exponent greater than 1.  In this
      case,

         gamma = encoding_exponent / end_to_end_exponent

      If you don't know whether the conditions under which the original
      image was captured (or calculated) warrant such a contrast change,
      you may assume that display intensities are proportional to
      original scene intensities; in other words, the end-to-end
      exponent is 1, so gamma and the encoding exponent are equal.

      If the image is being written to a file only, the encoder is free
      to choose the encoding exponent.  Choosing a value that causes the
      gamma value in the gAMA chunk to be 1/2.2 is often a reasonable
      choice because it minimizes the work for a decoder displaying on a
      typical video monitor.

      Some image renderers may simultaneously write the image to a PNG
      file and display it on-screen.  The displayed pixels should be
      appropriate for the display system, so that the user sees a proper
      representation of the intended scene.

      If the renderer wants to write the displayed sample values to the
      PNG file, avoiding a separate gamma encoding step for file output,
      then the renderer should approximate the transfer function of the
      display system by a power function, and write the reciprocal of
      the exponent into the gAMA chunk.  This will allow a PNG decoder
      to reproduce what the file's originator saw on screen during
      rendering.

      However, it is equally reasonable for a renderer to compute
      displayed pixels appropriate for the display device, and to
      perform separate gamma encoding for file storage, arranging to
      have a value in the gAMA chunk more appropriate to the future use
      of the image.

      Computer graphics renderers often do not perform gamma encoding,
      instead making sample values directly proportional to scene light
      intensity.  If the PNG encoder receives intensity samples that
      have already been quantized into integers, there is no point in
      doing gamma encoding on them; that would just result in further
      loss of information.  The encoder should just write the sample
      values to the PNG file.  This does not imply that the gAMA chunk
      should contain a gamma value of 1.0, because the desired end-to-
      end transfer function from scene intensity to display output
      intensity is not necessarily linear.  The desired gamma value is
      probably not far from 1.0, however.  It may depend on whether the
      scene being rendered is a daylight scene or an indoor scene, etc.

      When the sample values come directly from a piece of hardware, the
      correct gamma value can in principle be inferred from the transfer
      function of the hardware and the lighting conditions of the scene.
      In the case of video digitizers ("frame grabbers"), the samples
      are probably in the sRGB color space, because the sRGB
      specification was designed to be compatible with video standards.
      Image scanners are less predictable.  Their output samples may be
      proportional to the input light intensity because CCD (charge
      coupled device) sensors themselves are linear, or the scanner
      hardware may have already applied a power function designed to
      compensate for dot gain in subsequent printing (an exponent of
      about 0.57), or the scanner may have corrected the samples for
      display on a monitor.  The device documentation might describe the
      transformation performed, or might describe the target display or
      printer for the image data (which might be configurable).  You can
      also scan a calibrated target and use calibration software to
      determine the behavior of the device.  Remember that gamma relates
      file samples to desired display output, not to scanner input.

      File format converters generally should not attempt to convert
      supplied images to a different gamma.  Store the data in the PNG
      file without conversion, and deduce the gamma value from
      information in the source file if possible.  Gamma alteration at
      file conversion time causes re-quantization of the set of
      intensity levels that are represented, introducing further
      roundoff error with little benefit.  It's almost always better to
      just copy the sample values intact from the input to the output
      file.

      If the source file format describes the gamma characteristic of
      the image, a file format converter is strongly encouraged to write
      a PNG gAMA chunk.  Note that some file formats specify the
      exponent of the function mapping file samples to display output
      rather than the other direction.  If the source file's gamma value
      is greater than 1.0, it is probably a display system exponent, and
      you should use its reciprocal for the PNG gamma.  If the source
      file format records the relationship between image samples and
      something other than display output, then deducing the PNG gamma
      value will be more complex.

      Regardless of how an image was originally created, if an encoder
      or file format converter knows that the image has been displayed
      satisfactorily using a display system whose transfer function can
      be approximated by a power function with exponent
      display_exponent, then the image can be marked as having the gamma
      value:

         gamma = 1 / display_exponent

      It's better to write a gAMA chunk with an approximately right
      value than to omit the chunk and force PNG decoders to guess at an
      appropriate gamma.

      On the other hand, if the encoder has no way to infer the gamma
      value, then it is better to omit the gAMA chunk entirely.  If the
      image gamma has to be guessed at, leave it to the decoder to do
      the guessing.

      Gamma does not apply to alpha samples; alpha is always represented
      linearly.

      See also Recommendations for Decoders: Decoder gamma handling
      (Section 10.5).

   9.3. Encoder color handling

      See Color Tutorial (Chapter 14) if you aren't already familiar
      with color issues.

      Encoders capable of full-fledged color management [ICC] will
      perform more sophisticated calculations than those described here,
      and they may choose to use the iCCP chunk.  Encoders that know
      that their image samples conform to the sRGB specification [sRGB]
      are strongly encouraged to use the sRGB chunk.  Otherwise, this
      section applies.

      If it is possible for the encoder to determine the chromaticities
      of the source display primaries, or to make a strong guess based
      on the origin of the image or the hardware running it, then the
      encoder is strongly encouraged to output the cHRM chunk.  If it
      does so, the gAMA chunk should also be written; decoders can do
      little with cHRM if gAMA is missing.

      Video created with recent video equipment probably uses the CCIR
      709 primaries and D65 white point [ITU-R-BT709], which are:

                  R           G           B         White
         x      0.640       0.300       0.150       0.3127
         y      0.330       0.600       0.060       0.3290

      An older but still very popular video standard is SMPTE-C
      [SMPTE-170M]:

                  R           G           B         White
         x      0.630       0.310       0.155       0.3127
         y      0.340       0.595       0.070       0.3290

      The original NTSC color primaries have not been used in decades.
      Although you may still find the NTSC numbers listed in standards
      documents, you won't find any images that actually use them.

      Scanners that produce PNG files as output should insert the filter
      chromaticities into a cHRM chunk.

      In the case of hand-drawn or digitally edited images, you have to
      determine what monitor they were viewed on when being produced.
      Many image editing programs allow you to specify what type of
      monitor you are using.  This is often because they are working in
      some device-independent space internally.  Such programs have
      enough information to write valid cHRM and gAMA chunks, and should
      do so automatically.

      If the encoder is compiled as a portion of a computer image
      renderer that performs full-spectral rendering, the monitor values
      that were used to convert from the internal device-independent
      color space to RGB should be written into the cHRM chunk.  Any
      colors that are outside the gamut of the chosen RGB device should
      be clipped or otherwise constrained to be within the gamut; PNG
      does not store out-of-gamut colors.

      If the computer image renderer performs calculations directly in
      device-dependent RGB space, a cHRM chunk should not be written
      unless the scene description and rendering parameters have been
      adjusted to look good on a particular monitor.  In that case, the
      data for that monitor (if known) should be used to construct a
      cHRM chunk.

      There are often cases where an image's exact origins are unknown,
      particularly if it began life in some other format.  A few image
      formats store calibration information, which can be used to fill
      in the cHRM chunk.  For example, all PhotoCD images use the CCIR
      709 primaries and D65 whitepoint, so these values can be written
      into the cHRM chunk when converting a PhotoCD file.  PhotoCD also
      uses the SMPTE-170M transfer function.  (PhotoCD can store colors
      outside the RGB gamut, so the image data will require gamut
      mapping before writing to PNG format.)  TIFF 6.0 files can
      optionally store calibration information, which if present should
      be used to construct the cHRM chunk.  GIF and most other formats
      do not store any calibration information.

      It is not recommended that file format converters attempt to
      convert supplied images to a different RGB color space.  Store the
      data in the PNG file without conversion, and record the source
      primary chromaticities if they are known.  Color space
      transformation at file conversion time is a bad idea because of
      gamut mismatches and rounding errors.  As with gamma conversions,
      it's better to store the data losslessly and incur at most one
      conversion when the image is finally displayed.

      See also Recommendations for Decoders: Decoder color handling
      (Section 10.6).

   9.4. Alpha channel creation

      The alpha channel can be regarded either as a mask that
      temporarily hides transparent parts of the image, or as a means
      for constructing a non-rectangular image.  In the first case, the
      color values of fully transparent pixels should be preserved for
      future use.  In the second case, the transparent pixels carry no
      useful data and are simply there to fill out the rectangular image
      area required by PNG.  In this case, fully transparent pixels
      should all be assigned the same color value for best compression.

      Image authors should keep in mind the possibility that a decoder
      will ignore transparency control.  Hence, the colors assigned to
      transparent pixels should be reasonable background colors whenever
      feasible.

      For applications that do not require a full alpha channel, or
      cannot afford the price in compression efficiency, the tRNS
      transparency chunk is also available.

      If the image has a known background color, this color should be
      written in the bKGD chunk.  Even decoders that ignore transparency
      may use the bKGD color to fill unused screen area.

      If the original image has premultiplied (also called "associated")
      alpha data, convert it to PNG's non-premultiplied format by
      dividing each sample value by the corresponding alpha value, then
      multiplying by the maximum value for the image bit depth, and
      rounding to the nearest integer.  In valid premultiplied data, the
      sample values never exceed their corresponding alpha values, so
      the result of the division should always be in the range 0 to 1.
      If the alpha value is zero, output black (zeroes).

   9.5. Suggested palettes

      Suggested palettes can appear as sPLT chunks in any PNG file, or
      as a PLTE chunk in truecolor PNG files.  In either case, the
      suggested palette is not an essential part of the image data, but
      it may be used to present the image on indexed-color display
      hardware.  Suggested palettes are of no interest to viewers
      running on truecolor hardware.

      When sPLT is used to provide a suggested palette, it is
      recommended that the encoder use the frequency fields to indicate
      the relative importance of the palette entries, rather than leave
      them all zero (meaning undefined).  The frequency values are most
      easily computed as "nearest neighbor" counts, that is, the
      approximate usage of each RGBA palette entry if no dithering is
      applied.  (These counts will often be available for free as a
      consequence of developing the suggested palette.)  Because the
      suggested palette includes transparency information, it should be
      computed for the uncomposited image.

      Even for indexed-color images, sPLT can be used to define
      alternative reduced palettes for viewers that are unable to
      display all the colors present in the PLTE chunk.

      An older method for including a suggested palette in a truecolor
      PNG file uses the PLTE chunk.  If this method is used, the
      histogram (frequencies) should appear in a separate hIST chunk.
      Also, PLTE does not include transparency information, so for
      images of color type 6 (truecolor with alpha channel), it is
      recommended that a bKGD chunk appear and that the palette and
      histogram be computed with reference to the image as it would
      appear after compositing against the specified background color.
      This definition is necessary to ensure that useful palette entries
      are generated for pixels having fractional alpha values.  The
      resulting palette will probably be useful only to viewers that
      present the image against the same background color.  It is
      recommended that PNG editors delete or recompute the palette if
      they alter or remove the bKGD chunk in an image of color type 6.

      For images of color type 2 (truecolor without alpha channel), it
      is recommended that PLTE and hIST be computed with reference to
      the RGB data only, ignoring any transparent-color specification.
      If the file uses transparency (has a tRNS chunk), viewers can
      easily adapt the resulting palette for use with their intended
      background color.  They need only replace the palette entry
      closest to the tRNS color with their background color (which may
      or may not match the file's bKGD color, if any).

      If PLTE appears without bKGD in an image of color type 6, the
      circumstances under which the palette was computed are
      unspecified.

      For providing suggested palettes, sPLT is more flexible than PLTE
      in the following ways:

          * With sPLT, there can be multiple suggested palettes.  A
            decoder may choose an appropriate palette based on name or
            number of entries.

          * In an RGBA (color type 6) PNG, PLTE represents a palette
            already composited against the bKGD color, so it is useful
            only for display against that background color.  The sPLT
            chunk provides an uncomposited palette, which is useful for
            display against backgrounds of the decoder's choice.

          * Since sPLT is a noncritical chunk, a PNG editor can add or
            modify suggested palettes without being forced to discard
            unknown unsafe-to-copy chunks.

          * Whereas sPLT is allowed in PNG files of color types 0, 3,
            and 4 (grayscale and indexed), PLTE cannot be used to
            provide reduced palettes in these cases.

          * More than 256 entries can appear in sPLT.

      An encoder that uses sPLT may choose to write a PLTE/hIST
      suggested palette as well, for backward compatibility with
      decoders that do not recognize sPLT.

   9.6. Filter selection

      For images of color type 3 (indexed color), filter type 0 (None)
      is usually the most effective.  Note that color images with 256 or
      fewer colors should almost always be stored in indexed color
      format; truecolor format is likely to be much larger.

      Filter type 0 is also recommended for images of bit depths less
      than 8.  For low-bit-depth grayscale images, it may be a net win
      to expand the image to 8-bit representation and apply filtering,
      but this is rare.

      For truecolor and grayscale images, any of the five filters may
      prove the most effective.  If an encoder uses a fixed filter, the
      Paeth filter is most likely to be the best.

      For best compression of truecolor and grayscale images, we
      recommend an adaptive filtering approach in which a filter is
      chosen for each scanline.  The following simple heuristic has
      performed well in early tests: compute the output scanline using
      all five filters, and select the filter that gives the smallest
      sum of absolute values of outputs.  (Consider the output bytes as
      signed differences for this test.)  This method usually
      outperforms any single fixed filter choice.  However, it is likely
      that much better heuristics will be found as more experience is
      gained with PNG.

      Filtering according to these recommendations is effective on
      interlaced as well as noninterlaced images.

   9.7. Text chunk processing

      A nonempty keyword must be provided for each text chunk.  The
      generic keyword "Comment" can be used if no better description of
      the text is available.  If a user-supplied keyword is used, be
      sure to check that it meets the restrictions on keywords.

      PNG text strings are expected to use the Latin-1 character set.
      Encoders should avoid storing characters that are not defined in
      Latin-1, and should provide character code remapping if the local
      system's character set is not Latin-1.

      Encoders should discourage the creation of single lines of text
      longer than 79 characters, in order to facilitate easy reading.

      It is recommended that text items less than 1K (1024 bytes) in
      size should be output using uncompressed tEXt chunks.  In
      particular, it is recommended that the basic title and author
      keywords should always be output using uncompressed tEXt chunks.
      Lengthy disclaimers, on the other hand, are ideal candidates for
      zTXt.

      Placing large tEXt and zTXt chunks after the image data (after
      IDAT) can speed up image display in some situations, since the
      decoder won't have to read over the text to get to the image data.
      But it is recommended that small text chunks, such as the image
      title, appear before IDAT.

   9.8. Use of private chunks

      Applications can use PNG private chunks to carry information that
      need not be understood by other applications.  Such chunks must be
      given names with lowercase second letters, to ensure that they can
      never conflict with any future public chunk definition.  Note,
      however, that there is no guarantee that some other application
      will not use the same private chunk name.  If you use a private
      chunk type, it is prudent to store additional identifying
      information at the beginning of the chunk data.

      Use an ancillary chunk type (lowercase first letter), not a
      critical chunk type, for all private chunks that store information
      that is not absolutely essential to view the image.  Creation of
      private critical chunks is discouraged because they render PNG
      files unportable.  Such chunks should not be used in publicly
      available software or files.  If private critical chunks are
      essential for your application, it is recommended that one appear
      near the start of the file, so that a standard decoder need not
      read very far before discovering that it cannot handle the file.

      If you want others outside your organization to understand a chunk
      type that you invent, contact the maintainers of the PNG
      specification to submit a proposed chunk name and definition for
      addition to the list of special-purpose public chunks (see
      Additional chunk types, Section 4.4). Note that a proposed public
      chunk name (with uppercase second letter) must not be used in
      publicly available software or files until registration has been
      approved.

      If an ancillary chunk contains textual information that might be
      of interest to a human user, you should not create a special chunk
      type for it.  Instead use a tEXt chunk and define a suitable
      keyword.  That way, the information will be available to users not
      using your software.

      Keywords in tEXt chunks should be reasonably self-explanatory,
      since the idea is to let other users figure out what the chunk
      contains.  If of general usefulness, new keywords can be
      registered with the maintainers of the PNG specification.  But it
      is permissible to use keywords without registering them first.

   9.9. Private type and method codes

      This specification defines the meaning of only some of the
      possible values of some fields.  For example, only compression
      method 0 and filter types 0 through 4 are defined.  Numbers
      greater than 127 must be used when inventing experimental or
      private definitions of values for any of these fields.  Numbers
      below 128 are reserved for possible future public extensions of
      this specification.  Note that use of private type codes may
      render a file unreadable by standard decoders.  Such codes are
      strongly discouraged except for experimental purposes, and should
      not appear in publicly available software or files.

10. Recommendations for Decoders

   This chapter gives some recommendations for decoder behavior.  The
   only absolute requirement on a PNG decoder is that it successfully
   read any file conforming to the format specified in the preceding
   chapters.  However, best results will usually be achieved by
   following these recommendations.

   10.1. Error checking

      To ensure early detection of common file-transfer problems,
      decoders should verify that all eight bytes of the PNG file
      signature are correct.  (See Rationale: PNG file signature,
      Section 12.11.) A decoder can have additional confidence in the
      file's integrity if the next eight bytes are an IHDR chunk header
      with the correct chunk length.

      Unknown chunk types must be handled as described in Chunk naming
      conventions (Section 3.3).  An unknown chunk type is not to be
      treated as an error unless it is a critical chunk.

      It is strongly recommended that decoders should verify the CRC on
      each chunk.

      In some situations it is desirable to check chunk headers (length
      and type code) before reading the chunk data and CRC.  The chunk
      type can be checked for plausibility by seeing whether all four
      bytes are ASCII letters (codes 65-90 and 97-122); note that this
      need be done only for unrecognized type codes.  If the total file
      size is known (from file system information, HTTP protocol, etc),
      the chunk length can be checked for plausibility as well.

      If CRCs are not checked, dropped/added data bytes or an erroneous
      chunk length can cause the decoder to get out of step and
      misinterpret subsequent data as a chunk header.  Verifying that
      the chunk type contains letters is an inexpensive way of providing
      early error detection in this situation.

      For known-length chunks such as IHDR, decoders should treat an
      unexpected chunk length as an error.  Future extensions to this
      specification will not add new fields to existing chunks; instead,
      new chunk types will be added to carry new information.

      Unexpected values in fields of known chunks (for example, an
      unexpected compression method in the IHDR chunk) must be checked
      for and treated as errors.  However, it is recommended that
      unexpected field values be treated as fatal errors only in
      critical chunks.  An unexpected value in an ancillary chunk can be
      handled by ignoring the whole chunk as though it were an unknown
      chunk type.  (This recommendation assumes that the chunk's CRC has
      been verified.  In decoders that do not check CRCs, it is safer to
      treat any unexpected value as indicating a corrupted file.)

   10.2. Pixel dimensions

      Non-square pixels can be represented (see the pHYs chunk), but
      viewers are not required to account for them; a viewer can present
      any PNG file as though its pixels are square.

      Conversely, viewers running on display hardware with non-square
      pixels are strongly encouraged to rescale images for proper
      display.

   10.3. Truecolor image handling

      To achieve PNG's goal of universal interchangeability, decoders
      are required to accept all types of PNG image: indexed-color,
      truecolor, and grayscale.  Viewers running on indexed-color
      display hardware need to be able to reduce truecolor images to
      indexed format for viewing.  This process is usually called "color
      quantization".

      A simple, fast way of doing this is to reduce the image to a fixed
      palette.  Palettes with uniform color spacing ("color cubes") are
      usually used to minimize the per-pixel computation.  For
      photograph-like images, dithering is recommended to avoid ugly
      contours in what should be smooth gradients; however, dithering
      introduces graininess that can be objectionable.

      The quality of rendering can be improved substantially by using a
      palette chosen specifically for the image, since a color cube
      usually has numerous entries that are unused in any particular
      image.  This approach requires more work, first in choosing the
      palette, and second in mapping individual pixels to the closest
      available color.  PNG allows the encoder to supply suggested
      palettes, but not all encoders will do so, and the suggested
      palettes may be unsuitable in any case (they may have too many or
      too few colors).  High-quality viewers will therefore need to have
      a palette selection routine at hand.  A large lookup table is
      usually the most feasible way of mapping individual pixels to
      palette entries with adequate speed.

      Numerous implementations of color quantization are available.  The
      PNG reference implementation, libpng, includes code for the
      purpose.

   10.4. Sample depth rescaling

      Decoders may wish to scale PNG data to a lesser sample depth (data
      precision) for display.  For example, 16-bit data will need to be
      reduced to 8-bit depth for use on most present-day display
      hardware.  Reduction of 8-bit data to 5-bit depth is also common.

      The most accurate scaling is achieved by the linear equation

         output = ROUND(input * MAXOUTSAMPLE / MAXINSAMPLE)

      where

         MAXINSAMPLE = (2^sampledepth)-1
         MAXOUTSAMPLE = (2^desired_sampledepth)-1

      A slightly less accurate conversion is achieved by simply shifting
      right by sampledepth - desired_sampledepth places.  For example,
      to reduce 16-bit samples to 8-bit, one need only discard the low-
      order byte.  In many situations the shift method is sufficiently
      accurate for display purposes, and it is certainly much faster.
      (But if gamma correction is being done, sample rescaling can be
      merged into the gamma correction lookup table, as is illustrated
      in Decoder gamma handling, Section 10.5.)

      When an sBIT chunk is present, the original pre-PNG data can be
      recovered by shifting right to the sample depth specified by sBIT.
      Note that linear scaling will not necessarily reproduce the
      original data, because the encoder is not required to have used
      linear scaling to scale the data up.  However, the encoder is
      required to have used a method that preserves the high-order bits,
      so shifting always works.  This is the only case in which shifting
      might be said to be more accurate than linear scaling.

      When comparing pixel values to tRNS chunk values to detect
      transparent pixels, it is necessary to do the comparison exactly.
      Therefore, transparent pixel detection must be done before
      reducing sample precision.

   10.5. Decoder gamma handling

      See Gamma Tutorial (Chapter 13) if you aren't already familiar
      with gamma issues.

      Decoders capable of full-fledged color management [ICC] will
      perform more sophisticated calculations than what is described
      here.  Otherwise, this section applies.

      For an image display program to produce correct tone reproduction,
      it is necessary to take into account the relationship between file
      samples and display output, and the transfer function of the
      display system.  This can be done by calculating

         sample = integer_sample / (2^bitdepth - 1.0)
         display_output = sample ^ (1.0 / gamma)
         display_input = inverse_display_transfer(display_output)
         framebuf_sample = ROUND(display_input * MAX_FRAMEBUF_SAMPLE)

      where integer_sample is the sample value from the file,
      framebuf_sample is the value to write into the frame buffer, and
      MAX_FRAMEBUF_SAMPLE is the maximum value of a frame buffer sample
      (255 for 8-bit, 31 for 5-bit, etc).  The first line converts an
      integer sample into a normalized 0-to-1 floating-point value, the
      second converts to a value proportional to the desired display
      output intensity, the third accounts for the display system's
      transfer function, and the fourth converts to an integer frame
      buffer sample.

      A step could be inserted between the second and third to adjust
      display_output to account for the difference between the actual
      viewing conditions and the reference viewing conditions.  However,
      this adjustment requires accounting for veiling glare, black
      mapping, and color appearance models, none of which can be well
      approximated by power functions.  The calculations are not
      described here.  If viewing conditions are ignored, the error will
      usually be small.

      Typically, the display transfer function can be approximated by a
      power function with exponent display_exponent, in which case the
      second and third lines can be merged into

         display_input = sample ^ (1.0 / (gamma * display_exponent))
                       = sample ^ decoding_exponent

      so as to perform only one power calculation.  For color images,
      the entire calculation is performed separately for R, G, and B
      values.

      The value of gamma can be taken directly from the gAMA chunk.
      Alternatively, an application may wish to allow the user to adjust
      the appearance of the displayed image by influencing the value of
      gamma.  For example, the user could manually set a parameter
      called user_exponent that defaults to 1.0, and the application
      could set

         gamma = gamma_from_file / user_exponent
         decoding_exponent = 1.0 / (gamma * display_exponent)
               = user_exponent / (gamma_from_file * display_exponent)

      The user would set user_exponent greater than 1 to darken the
      mid-level tones, or less than 1 to lighten them.

      It is not necessary to perform transcendental math for every
      pixel.  Instead, compute a lookup table that gives the correct
      output value for every possible sample value.  This requires only
      256 calculations per image (for 8-bit accuracy), not one or three
      calculations per pixel.  For an indexed-color image, a one-time
      correction of the palette is sufficient, unless the image uses
      transparency and is being displayed against a nonuniform
      background.

      If floating-point calculations are not possible, gamma correction
      tables can be computed using integer arithmetic and a precomputed
      table of logarithms.  Example code appears in the "Extensions to
      the PNG Specification" document [PNG-EXTENSIONS].

      When the incoming image has unknown gamma (gAMA, sRGB, and iCCP
      all absent), choose a likely default gamma value, but allow the
      user to select a new one if the result proves too dark or too
      light.  The default gamma can depend on other knowledge about the
      image, like whether it came from the Internet or from the local
      system.

      In practice, it is often difficult to determine what value of the
      display exponent should be used.  In systems with no built-in
      gamma correction, the display exponent is determined entirely by
      the CRT (cathode ray tube).  Assume a CRT exponent of 2.2 unless
      detailed calibration measurements of this particular CRT are
      available.

      Many modern frame buffers have lookup tables that are used to
      perform gamma correction, and on these systems the display
      exponent value should be the exponent of the lookup table and CRT
      combined.  You may not be able to find out what the lookup table
      contains from within an image viewer application, so you may have
      to ask the user what the display system's exponent is.
      Unfortunately, different manufacturers use different ways of
      specifying what should go into the lookup table, so interpretation
      of the system "gamma" value is system-dependent.  The Gamma
      Tutorial (Chapter 13) gives some examples.

      The response of real displays is actually more complex than can be
      described by a single number (the display exponent).  If actual
      measurements of the monitor's light output as a function of
      voltage input are available, the third and fourth lines of the
      computation above can be replaced by a lookup in these
      measurements, to find the actual frame buffer value that most
      nearly gives the desired brightness.

   10.6. Decoder color handling

      See Color Tutorial (Chapter 14) if you aren't already familiar
      with color issues.

      In many cases, decoders will treat image data in PNG files as
      device-dependent RGB data and display it without modification
      (except for appropriate gamma correction).  This provides the
      fastest display of PNG images.  But unless the viewer uses exactly
      the same display hardware as the original image author used, the
      colors will not be exactly the same as the original author saw,
      particularly for darker or near-neutral colors.  The cHRM chunk
      provides information that allows closer color matching than that
      provided by gamma correction alone.

      Decoders can use the cHRM data to transform the image data from
      RGB to CIE XYZ and thence into a perceptually linear color space
      such as CIE LAB.  They can then partition the colors to generate
      an optimal palette, because the geometric distance between two
      colors in CIE LAB is strongly related to how different those
      colors appear (unlike, for example, RGB or XYZ spaces).  The
      resulting palette of colors, once transformed back into RGB color
      space, could be used for display or written into a PLTE chunk.

      Decoders that are part of image processing applications might also
      transform image data into CIE LAB space for analysis.

      In applications where color fidelity is critical, such as product
      design, scientific visualization, medicine, architecture, or
      advertising, decoders can transform the image data from source RGB
      to the display RGB space of the monitor used to view the image.
      This involves calculating the matrix to go from source RGB to XYZ
      and the matrix to go from XYZ to display RGB, then combining them
      to produce the overall transformation.  The decoder is responsible
      for implementing gamut mapping.

      Decoders running on platforms that have a Color Management System
      (CMS) can pass the image data, gAMA, and cHRM values to the CMS
      for display or further processing.

      Decoders that provide color printing facilities can use the
      facilities in Level 2 PostScript to specify image data in
      calibrated RGB space or in a device-independent color space such
      as XYZ.  This will provide better color fidelity than a simple RGB
      to CMYK conversion.  The PostScript Language Reference manual
      [POSTSCRIPT] gives examples.  Such decoders are responsible for
      implementing gamut mapping between source RGB (specified in the
      cHRM chunk) and the target printer.  The PostScript interpreter is
      then responsible for producing the required colors.

      Decoders can use the cHRM data to calculate an accurate grayscale
      representation of a color image.  Conversion from RGB to gray is
      simply a case of calculating the Y (luminance) component of XYZ,
      which is a weighted sum of the R, G, and B values.  The weights
      depend on the monitor type, i.e., the values in the cHRM chunk.
      Decoders may wish to do this for PNG files with no cHRM chunk.  In
      that case, a reasonable default would be the CCIR 709 primaries
      [ITU-R-BT709]. Do not use the original NTSC primaries, unless you
      really do have an image color-balanced for such a monitor.  Few
      monitors ever used the NTSC primaries, so such images are probably
      nonexistent these days.

   10.7. Background color

      The background color given by bKGD will typically be used to fill
      unused screen space around the image, as well as any transparent
      pixels within the image.  (Thus, bKGD is valid and useful even
      when the image does not use transparency.)  If no bKGD chunk is
      present, the viewer will need to make its own decision about a
      suitable background color.

      Viewers that have a specific background against which to present
      the image (such as Web browsers) should ignore the bKGD chunk, in
      effect overriding bKGD with their preferred background color or
      background image.

      The background color given by bKGD is not to be considered
      transparent, even if it happens to match the color given by tRNS
      (or, in the case of an indexed-color image, refers to a palette
      index that is marked as transparent by tRNS).  Otherwise one would
      have to imagine something "behind the background" to composite
      against.  The background color is either used as background or
      ignored; it is not an intermediate layer between the PNG image and
      some other background.

      Indeed, it will be common that bKGD and tRNS specify the same
      color, since then a decoder that does not implement transparency
      processing will give the intended display, at least when no
      partially-transparent pixels are present.

   10.8. Alpha channel processing

      In the most general case, the alpha channel can be used to
      composite a foreground image against a background image; the PNG
      file defines the foreground image and the transparency mask, but
      not the background image.  Decoders are not required to support
      this most general case.  It is expected that most will be able to
      support compositing against a single background color, however.

      The equation for computing a composited sample value is

         output = alpha * foreground + (1-alpha) * background

      where the alpha value and the input and output sample values are
      expressed as fractions in the range 0 to 1.  This computation
      should be performed with intensity samples (not gamma-encoded
      samples).  For color images, the computation is done separately
      for R, G, and B samples.

      The following code illustrates the general case of compositing a
      foreground image over a background image.  It assumes that you
      have the original pixel data available for the background image,
      and that output is to a frame buffer for display.  Other variants
      are possible; see the comments below the code.  The code allows
      the sample depths and gamma values of foreground and background
      images to be different, and not necessarily suited to the display
      system.  Don't assume everything is the same without checking.

      This code is standard C, with line numbers added for reference in
      the comments below:

         01  int foreground[4];  /* image pixel: R, G, B, A */
         02  int background[3];  /* background pixel: R, G, B */
         03  int fbpix[3];       /* frame buffer pixel */
         04  int fg_maxsample;   /* foreground max sample */
         05  int bg_maxsample;   /* background max sample */
         06  int fb_maxsample;   /* frame buffer max sample */
         07  int ialpha;
         08  float alpha, compalpha;
         09  float gamfg, linfg, gambg, linbg, comppix, gcvideo;

             /* Get max sample values in data and frame buffer */
         10  fg_maxsample = (1 << fg_sample_depth) - 1;
         11  bg_maxsample = (1 << bg_sample_depth) - 1;
         12  fb_maxsample = (1 << frame_buffer_sample_depth) - 1;
             /*
              * Get integer version of alpha.
              * Check for opaque and transparent special cases;
              * no compositing needed if so.
              *
              * We show the whole gamma decode/correct process in
              * floating point, but it would more likely be done
              * with lookup tables.
              */
         13  ialpha = foreground[3];

         14  if (ialpha == 0) {
                 /*
                  * Foreground image is transparent here.
                  * If the background image is already in the frame
                  * buffer, there is nothing to do.
                  */
         15      ;
         16  } else if (ialpha == fg_maxsample) {
                 /*
                  * Copy foreground pixel to frame buffer.
                  */
         17      for (i = 0; i < 3; i++) {
         18          gamfg = (float) foreground[i] / fg_maxsample;
         19          linfg = pow(gamfg, 1.0/fg_gamma);
         20          comppix = linfg;
         21          gcvideo = pow(comppix, 1.0/display_exponent);
         22          fbpix[i] = (int) (gcvideo * fb_maxsample + 0.5);
         23      }
         24  } else {
                 /*
                  * Compositing is necessary.
                  * Get floating-point alpha and its complement.
                  * Note: alpha is always linear; gamma does not
                  * affect it.
                  */
         25      alpha = (float) ialpha / fg_maxsample;
         26      compalpha = 1.0 - alpha;

         27      for (i = 0; i < 3; i++) {
                     /*
                      * Convert foreground and background to
                      * floating point, then undo gamma encoding.
                      */
         28          gamfg = (float) foreground[i] / fg_maxsample;
         29          linfg = pow(gamfg, 1.0/fg_gamma);
         30          gambg = (float) background[i] / bg_maxsample;
         31          linbg = pow(gambg, 1.0/bg_gamma);
                     /*
                      * Composite.
                      */
         32          comppix = linfg * alpha + linbg * compalpha;
                     /*
                      * Gamma correct for display.
                      * Convert to integer frame buffer pixel.
                      */
         33          gcvideo = pow(comppix, 1.0/display_exponent);
         34          fbpix[i] = (int) (gcvideo * fb_maxsample + 0.5);
         35      }
         36  }

      Variations:

         1. If output is to another PNG image file instead of a frame
            buffer, lines 21, 22, 33, and 34 should be changed to be
            something like:

               /*
                * Gamma encode for storage in output file.
                * Convert to integer sample value.
                */
               gamout = pow(comppix, outfile_gamma);
               outpix[i] = (int) (gamout * out_maxsample + 0.5);

            Also, it becomes necessary to process background pixels when
            alpha is zero, rather than just skipping pixels.  Thus, line
            15 will need to be replaced by copies of lines 17-23, but
            processing background instead of foreground pixel values.

         2. If the sample depths of the output file, foreground file,
            and background file are all the same, and the three gamma
            values also match, then the no-compositing code in lines
            14-23 reduces to nothing more than copying pixel values from
            the input file to the output file if alpha is one, or
            copying pixel values from background to output file if alpha
            is zero.  Since alpha is typically either zero or one for
            the vast majority of pixels in an image, this is a great
            savings.  No gamma computations are needed for most pixels.

         3. When the sample depths and gamma values all match, it may
            appear attractive to skip the gamma decoding and encoding
            (lines 28-31, 33-34) and just perform line 32 using gamma-
            encoded sample values.  Although this doesn't hurt image
            quality too badly, the time savings are small if alpha
            values of zero and one are special-cased as recommended
            here.

         4. If the original pixel values of the background image are no
            longer available, only processed frame buffer pixels left by
            display of the background image, then lines 30 and 31 need
            to extract intensity from the frame buffer pixel values
            using code like:

               /*
                * Convert frame buffer value into intensity sample.
                */
               gcvideo = (float) fbpix[i] / fb_maxsample;
               linbg = pow(gcvideo, display_exponent);

            However, some roundoff error can result, so it is better to
            have the original background pixels available if at all
            possible.

         5. Note that lines 18-22 are performing exactly the same gamma
            computation that is done when no alpha channel is present.
            So, if you handle the no-alpha case with a lookup table, you
            can use the same lookup table here.  Lines 28-31 and 33-34
            can also be done with (different) lookup tables.

         6. Of course, everything here can be done in integer
            arithmetic.  Just be careful to maintain sufficient
            precision all the way through.

      Note: in floating-point arithmetic, no overflow or underflow
      checks are needed, because the input sample values are guaranteed
      to be between 0 and 1, and compositing always yields a result that
      is in between the input values (inclusive).  With integer
      arithmetic, some roundoff-error analysis might be needed to
      guarantee no overflow or underflow.

      When displaying a PNG image with full alpha channel, it is
      important to be able to composite the image against some
      background, even if it's only black.  Ignoring the alpha channel
      will cause PNG images that have been converted from an
      associated-alpha representation to look wrong.  (Of course, if the
      alpha channel is a separate transparency mask, then ignoring alpha
      is a useful option: it allows the hidden parts of the image to be
      recovered.)

      Even if the decoder author does not wish to implement true
      compositing logic, it is simple to deal with images that contain
      only zero and one alpha values.  (This is implicitly true for
      grayscale and truecolor PNG files that use a tRNS chunk; for
      indexed-color PNG files, it is easy to check whether tRNS contains
      any values other than 0 and 255.)  In this simple case,
      transparent pixels are replaced by the background color, while
      others are unchanged.

      If a decoder contains only this much transparency capability, it
      should deal with a full alpha channel by converting it to a binary
      alpha channel, either by treating all nonzero alpha values as
      fully opaque or by dithering.  Neither approach will yield very
      good results for images converted from associated-alpha formats,
      but it's better than doing nothing.  Dithering full alpha to
      binary alpha is very much like dithering grayscale to black-and-
      white, except that all fully transparent and fully opaque pixels
      should be left unchanged by the dither.

   10.9. Progressive display

      When receiving images over slow transmission links, decoders can
      improve perceived performance by displaying interlaced images
      progressively.  This means that as each pass is received, an
      approximation to the complete image is displayed based on the data
      received so far.  One simple yet pleasing effect can be obtained
      by expanding each received pixel to fill a rectangle covering the
      yet-to-be-transmitted pixel positions below and to the right of
      the received pixel.  This process can be described by the
      following pseudocode:

         Starting_Row [1..7] =  { 0, 0, 4, 0, 2, 0, 1 }
         Starting_Col [1..7] =  { 0, 4, 0, 2, 0, 1, 0 }
         Row_Increment [1..7] = { 8, 8, 8, 4, 4, 2, 2 }
         Col_Increment [1..7] = { 8, 8, 4, 4, 2, 2, 1 }
         Block_Height [1..7] =  { 8, 8, 4, 4, 2, 2, 1 }
         Block_Width [1..7] =   { 8, 4, 4, 2, 2, 1, 1 }

         pass := 1
         while pass <= 7
         begin
             row := Starting_Row[pass]

             while row < height
             begin
                 col := Starting_Col[pass]

                 while col < width
                 begin
                     visit (row, col,
                            min (Block_Height[pass], height - row),
                            min (Block_Width[pass], width - col))
                     col := col + Col_Increment[pass]
                 end
                 row := row + Row_Increment[pass]
             end

             pass := pass + 1
         end

      Here, the function visit(row,column,height,width) obtains the next
      transmitted pixel and paints a rectangle of the specified height
      and width, whose upper-left corner is at the specified row and
      column, using the color indicated by the pixel.  Note that row and
      column are measured from 0,0 at the upper left corner.

      If the decoder is merging the received image with a background
      image, it may be more convenient just to paint the received pixel
      positions; that is, the visit() function sets only the pixel at
      the specified row and column, not the whole rectangle.  This
      produces a "fade-in" effect as the new image gradually replaces
      the old.  An advantage of this approach is that proper alpha or
      transparency processing can be done as each pixel is replaced.
      Painting a rectangle as described above will overwrite
      background-image pixels that may be needed later, if the pixels
      eventually received for those positions turn out to be wholly or
      partially transparent.  Of course, this is a problem only if the
      background image is not stored anywhere offscreen.

   10.10. Suggested-palette and histogram usage

      For viewers running on indexed-color hardware trying to display a
      truecolor image, or an indexed-color image whose palette is too
      large for the framebuffer, the encoder may have provided one or
      more suggested palettes in sPLT chunks.  If one of them is found
      to be suitable, based on its size and perhaps its name, the
      decoder can use that palette.  Note that suggested palettes with a
      sample depth different from what the decoder needs can be
      converted using sample depth rescaling (See Recommendations for
      Decoders: Sample depth rescaling, Section 10.4).

      When the background is a solid color, the decoder should composite
      the image and the suggested palette against that color, then
      quantize the resulting image to the resulting RGB palette.  When
      the image uses transparency and the background is not a solid
      color, no suggested palette is likely to be useful.

      For truecolor images, a suggested palette might also be provided
      in a PLTE chunk.  If the image has a tRNS chunk and the background
      is a solid color, the viewer can adapt the suggested paletted for
      use with this background color.  To do this, replace the palette
      entry closest to the tRNS color with the background color, or just
      add a palette entry for the background color if the viewer can
      handle more colors than there are palette entries.

      For images of color type 6 (truecolor with alpha channel), any
      PLTE chunk should have been designed for display of the image
      against a uniform background of the color specified by bKGD.
      Viewers should probably ignore the palette if they intend to use a
      different background, or if the bKGD chunk is missing.  Viewers
      can use the suggested palette for display against a different
      background than it was intended for, but the results may not be
      very good.

      If the viewer presents a transparent truecolor image against a
      background that is more complex than a single color, it is
      unlikely that the PLTE chunk will be optimal for the composite
      image.  In this case it is best to perform a truecolor compositing
      step on the truecolor PNG image and background image, then color-
      quantize the resulting image.

      In truecolor PNG files, if both PLTE and sPLT appear, the decoder
      can choose from among the palettes suggested by both, bearing in
      mind the different transparency semantics mentioned above.

      The frequencies in sPLT and hIST chunks are useful when the viewer
      cannot provide as many colors as are used in the palette.  If the
      viewer is short only a few colors, it is usually adequate to drop
      the least-used colors from the palette.  To reduce the number of
      colors substantially, it's best to choose entirely new
      representative colors, rather than trying to use a subset of the
      existing palette.  This amounts to performing a new color
      quantization step; however, the existing palette and frequencies
      can be used as the input data, thus avoiding a scan of the image
      data.

      If no suggested palettes are provided, a decoder can develop its
      own, at the cost of an extra pass over the image data.
      Alternatively, a default palette (probably a color cube) can be
      used.

      See also Recommendations for Encoders: Suggested palettes (Section
      9.5).

   10.11. Text chunk processing

      If practical, decoders should have a way to display to the user
      all tEXt and zTXt chunks found in the file.  Even if the decoder
      does not recognize a particular text keyword, the user might be
      able to understand it.

      PNG text is not supposed to contain any characters outside the ISO
      8859-1 (Latin-1) character set (that is, no codes 0-31 or
      127-159), except for the newline character (decimal 10).  But
      decoders might encounter such characters anyway.  Some of these
      characters can be safely displayed (e.g., TAB, FF, and CR, decimal
      9, 12, and 13, respectively), but others, especially the ESC
      character (decimal 27), could pose a security hazard because
      unexpected actions may be taken by display hardware or software.
      To prevent such hazards, decoders should not attempt to directly
      display any non-Latin-1 characters (except for newline and perhaps
      TAB, FF, CR) encountered in a tEXt or zTXt chunk.  Instead, ignore
      them or display them in a visible notation such as "\nnn".  See
      Security considerations (Section 8.5).

      Even though encoders are supposed to represent newlines as LF, it
      is recommended that decoders not rely on this; it's best to
      recognize all the common newline combinations (CR, LF, and CR-LF)
      and display each as a single newline.  TAB can be expanded to the
      proper number of spaces needed to arrive at a column multiple of
      8.

      Decoders running on systems with non-Latin-1 character set
      encoding should provide character code remapping so that Latin-1
      characters are displayed correctly.  Some systems may not provide
      all the characters defined in Latin-1.  Mapping unavailable
      characters to a visible notation such as "\nnn" is a good
      fallback.  In particular, character codes 127-255 should be
      displayed only if they are printable characters on the decoding
      system.  Some systems may interpret such codes as control
      characters; for security, decoders running on such systems should
      not display such characters literally.

      Decoders should be prepared to display text chunks that contain
      any number of printing characters between newline characters, even
      though encoders are encouraged to avoid creating lines in excess
      of 79 characters.

11. Glossary

   a^b
      Exponentiation; a raised to the power b.  Note that zero raised to
      any positive power is zero.  C programmers should be careful not
      to misread this notation as exclusive-or.

   Alpha
      A value representing a pixel's degree of transparency.  The more
      transparent a pixel, the less it hides the background against
      which the image is presented.  In PNG, alpha is really the degree
      of opacity:  zero alpha represents a completely transparent pixel,
      maximum alpha represents a completely opaque pixel.  But most
      people refer to alpha as providing transparency information, not
      opacity information, and we continue that custom here.

   Ancillary chunk
      A chunk that provides additional information.  A decoder can still
      produce a meaningful image, though not necessarily the best
      possible image, without processing the chunk.

   Bit depth
      The number of bits per palette index (in indexed-color PNGs) or
      per sample (in other color types).  This is the same value that
      appears in IHDR.

   Byte
      Eight bits; also called an octet.

   Channel
      The set of all samples of the same kind within an image; for
      example, all the blue samples in a truecolor image.  (The term
      "component" is also used, but not in this specification.)  A
      sample is the intersection of a channel and a pixel.

   Chromaticity
      A pair of values x,y that precisely specify a color, except for
      the brightness information.

   Chunk
      A section of a PNG file.  Each chunk has a type indicated by its
      chunk type name.  Most types of chunks also include some data.
      The format and meaning of the data within the chunk are determined
      by the type name.

   CIE
      International Commission on Illumination (Commission
      Internationale de l'Eclairage).

   CIE XYZ
      A device-independent color space in which each component is the
      sum of a weighted power distribution over the visible spectrum.
      The Y component is luminence (see below).

   CIE LAB
      A perceptually linear color space.

   Composite
      As a verb, to form an image by merging a foreground image and a
      background image, using transparency information to determine
      where the background should be visible.  The foreground image is
      said to be "composited against" the background.

   CRC
      Cyclic Redundancy Check.  A CRC is a type of check value designed
      to catch most transmission errors.  A decoder calculates the CRC
      for the received data and compares it to the CRC that the encoder
      calculated, which is appended to the data.  A mismatch indicates
      that the data was corrupted in transit.

   Critical chunk
      A chunk that must be understood and processed by the decoder in
      order to produce a meaningful image from a PNG file.

   CRT
      Cathode Ray Tube: a common type of computer display hardware.

   Datastream
      A sequence of bytes.  This term is used rather than "file" to
      describe a byte sequence that is only a portion of a file.  We
      also use it to emphasize that a PNG image might be generated and
      consumed "on-the-fly", never appearing in a stored file at all.

   Deflate
      The name of the compression algorithm used in standard PNG files,
      as well as in zip, gzip, pkzip, and other compression programs.
      Deflate is a member of the LZ77 family of compression methods.

   Filter
      A transformation applied to image data in hopes of improving its
      compressibility.  PNG uses only lossless (reversible) filter
      algorithms.

   Frame buffer
      The final digital storage area for the image shown by a computer
      display.  Software causes an image to appear onscreen by loading
      it into the frame buffer.

   Gamma
      Informally, a measure of the brightness of mid-level tones in an
      image.  Outside this specification, the term "gamma" is often used
      as the exponent of a power function that is the transfer function
      of any stage(s) of an imaging pipeline:

         output = input ^ gamma

      where both input and output are scaled to the range 0 to 1.
      Within this specification, gamma refers specifically to the
      function from display output to image samples.

   Grayscale
      An image representation in which each pixel is represented by a
      single sample value representing overall luminance (on a scale
      from black to white).  PNG also permits an alpha sample to be
      stored for each pixel of a grayscale image.

   Indexed color
      An image representation in which each pixel is represented by a
      single sample that is an index into a palette or lookup table.
      The selected palette entry defines the actual color of the pixel.

   Intensity
      Power per unit area of light entering or leaving a surface.  It is
      often normalized to the range 0 to 1 by dividing by a maximum
      intensity.

   Lossless compression
      Any method of data compression that guarantees the original data
      can be reconstructed exactly, bit-for-bit.

   Lossy compression
      Any method of data compression that reconstructs the original data
      approximately, rather than exactly.

   LSB
      Least Significant Byte of a multi-byte value.

   Luminance
      Perceived brightness, or grayscale level, of a color.  Luminance
      and chromaticity together fully define a perceived color.

   LUT
      Look Up Table.  In general, a table used to transform data.  In
      frame buffer hardware, a LUT can be used to map indexed-color
      pixels into a selected set of truecolor values, or to perform
      gamma correction.  In software, a LUT can be used as a fast way of
      implementing any one-variable mathematical function.

   MSB
      Most Significant Byte of a multi-byte value.

   Palette
      The set of colors available in an indexed-color image.  In PNG, a
      palette is an array of colors defined by red, green, and blue
      samples.  (Alpha values can also be defined for palette entries,
      via the tRNS chunk.)

   Pixel
      The information stored for a single grid point in the image.  The
      complete image is a rectangular array of pixels.

   PNG editor
      A program that modifies a PNG file and preserves ancillary
      information, including chunks that it does not recognize.  Such a
      program must obey the rules given in Chunk Ordering Rules (Chapter
      7).

   Sample
      A single number in the image data; for example, the red value of a
      pixel.  A pixel is composed of one or more samples.  When
      discussing physical data layout (in particular, in Image layout,
      Section 2.3), we use "sample" to mean a number stored in the image
      array.  It would be more precise but much less readable to say
      "sample or palette index" in that context.  Elsewhere in the
      specification, "sample" means a color value or alpha value.  In
      the indexed-color case, these are palette entries not palette
      indexes.

   Sample depth
      The precision, in bits, of color values and alpha values.  In
      indexed-color PNGs the sample depth is always 8 by definition of
      the PLTE chunk.  In other color types it is the same as the bit
      depth.

   Scanline
      One horizontal row of pixels within an image.

   Truecolor
      An image representation in which pixel colors are defined by
      storing three samples for each pixel, representing red, green, and
      blue intensities respectively.  PNG also permits an alpha sample
      to be stored for each pixel of a truecolor image.

   White point
      The chromaticity of a computer display's nominal white value.

   zlib
      A particular format for data that has been compressed using
      deflate-style compression.  Also the name of a library
      implementing this method.  PNG implementations need not use the
      zlib library, but they must conform to its format for compressed
      data.

12. Appendix: Rationale

   (This appendix is not part of the formal PNG specification.)

   This appendix gives the reasoning behind some of the design decisions
   in PNG.  Many of these decisions were the subject of considerable
   debate.  The authors freely admit that another group might have made
   different decisions; however, we believe that our choices are
   defensible and consistent.

   12.1. Why a new file format?

      Does the world really need yet another graphics format?  We
      believe so.  GIF is no longer freely usable, but no other commonly
      used format can directly replace it, as is discussed in more
      detail below.  We might have used an adaptation of an existing
      format, for example GIF with an unpatented compression scheme.
      But this would require new code anyway; it would not be all that
      much easier to implement than a whole new file format.  (PNG is
      designed to be simple to implement, with the exception of the
      compression engine, which would be needed in any case.)  We feel
      that this is an excellent opportunity to design a new format that
      fixes some of the known limitations of GIF.

   12.2. Why these features?

      The features chosen for PNG are intended to address the needs of
      applications that previously used the special strengths of GIF.
      In particular, GIF is well adapted for online communications
      because of its streamability and progressive display capability.
      PNG shares those attributes.

      We have also addressed some of the widely known shortcomings of
      GIF.  In particular, PNG supports truecolor images.  We know of no
      widely used image format that losslessly compresses truecolor
      images as effectively as PNG does.  We hope that PNG will make use
      of truecolor images more practical and widespread.

      Some form of transparency control is desirable for applications in
      which images are displayed against a background or together with
      other images.  GIF provided a simple transparent-color
      specification for this purpose.  PNG supports a full alpha channel
      as well as transparent-color specifications.  This allows both
      highly flexible transparency and compression efficiency.

      Robustness against transmission errors has been an important
      consideration.  For example, images transferred across the
      Internet are often mistakenly processed as text, leading to file
      corruption.  PNG is designed so that such errors can be detected
      quickly and reliably.

      PNG has been expressly designed not to be completely dependent on
      a single compression technique.  Although deflate/inflate
      compression is mentioned in this document, PNG would still exist
      without it.

   12.3. Why not these features?

      Some features have been deliberately omitted from PNG.  These
      choices were made to simplify implementation of PNG, promote
      portability and interchangeability, and make the format as simple
      and foolproof as possible for users.  In particular:

          * There is no uncompressed variant of PNG.  It is possible to
            store uncompressed data by using only uncompressed deflate
            blocks (a feature normally used to guarantee that deflate
            does not make incompressible data much larger).  However,
            PNG software must support full deflate/inflate; any software
            that does not is not compliant with the PNG standard.  The
            two most important features of PNG--portability and
            compression--are absolute requirements for online
            applications, and users demand them.  Failure to support
            full deflate/inflate compromises both of these objectives.

          * There is no lossy compression in PNG.  Existing formats such
            as JFIF (JPEG File Interchange Format) already handle lossy
            compression well.  Furthermore, available lossy compression
            methods, e.g., the JPEG (Joint Photographic Experts Group)
            algorithm, are far from foolproof--a poor choice of quality
            level can ruin an image.  To avoid user confusion and
            unintentional loss of information, we feel it is best to
            keep lossy and lossless formats strictly separate.  Also,
            lossy compression is complex to implement.  Adding JPEG
            support to a PNG decoder might significantly increase its
            size, causing some decoders to omit support for the feature,
            which would erode our goal of interchangeability.

          * There is no support for CMYK (Cyan, Magenta, Yellow, blacK)
            or other unusual color spaces.  Again, this is in the name
            of promoting portability.  CMYK, in particular, is far too
            device-dependent to be useful as a portable image
            representation.

          * There is no standard chunk for thumbnail views of images.
            In discussions with software vendors who use thumbnails in
            their products, it has become clear that most would not use
            a "standard" thumbnail chunk.  For one thing, every vendor
            has a different idea of what the dimensions and
            characteristics of a thumbnail ought to be.  Also, some
            vendors keep thumbnails in separate files to accommodate
            varied image formats; they are not going to stop doing that
            simply because of a thumbnail chunk in one new format.
            Proprietary chunks containing vendor-specific thumbnails
            appear to be more practical than a common thumbnail format.

      It is worth noting that private extensions to PNG could easily add
      these features.  We will not, however, include them as part of the
      basic PNG standard.

      PNG also does not support multiple images in one file.  This
      restriction is a reflection of the reality that many applications
      do not need and will not support multiple images per file.  In any
      case, single images are a fundamentally different sort of object
      from sequences of images.  Rather than make false promises of
      interchangeability, we have drawn a clear distinction between
      single-image and multi-image formats.  PNG is a single-image
      format.  (But see Multiple-image extension, Section 8.4.)

   12.4. Why not use format X?

      We considered numerous existing formats before deciding to develop
      PNG.  None could meet the requirements that we felt were important
      for PNG.

      GIF is no longer suitable as a universal standard because of legal
      entanglements.  Although just replacing GIF's compression method
      would avoid that problem, GIF does not support truecolor images,
      alpha channels, or gamma correction.  The spec has more subtle
      problems too.  Only a small subset of the GIF89 spec is actually
      portable across a variety of implementations, but there is no
      codification of the most portable part of the spec.

      TIFF (the Tagged Image File Format) is far too complex to meet our
      goals of simplicity and interchangeability.  Defining a TIFF
      subset would meet that objection, but would frustrate users making
      the reasonable assumption that a file saved as TIFF from their
      existing software would load into a program supporting our flavor
      of TIFF.  Furthermore, TIFF is not designed for stream processing,
      has no provision for progressive display, and does not currently
      provide any good, legally unencumbered, lossless compression
      method.

      IFF has also been suggested, but is not suitable in detail:
      available image representations are too machine-specific or not
      adequately compressed.  The overall chunk structure of IFF is a
      useful concept that PNG has liberally borrowed from, but we did
      not attempt to be bit-for-bit compatible with IFF chunk structure.
      Again this is due to detailed issues, notably the fact that IFF
      FORMs are not designed to be serially writable.

      Lossless JPEG is not suitable because it does not provide for the
      storage of indexed-color images.  Furthermore, its lossless
      truecolor compression is often inferior to that of PNG.

   12.5. Byte order

      It has been asked why PNG uses network byte order.  We have
      selected one byte ordering and used it consistently.  Which order
      in particular is of little relevance, but network byte order has
      the advantage that routines to convert to and from it are already
      available on any platform that supports TCP/IP networking,
      including all PC platforms.  The functions are trivial and will be
      included in the reference implementation.

   12.6. Interlacing

      PNG's two-dimensional interlacing scheme is more complex to
      implement than GIF's line-wise interlacing.  It also costs a
      little more in file size.  However, it yields an initial image
      eight times faster than GIF (the first pass transmits only 1/64th
      of the pixels, compared to 1/8th for GIF).  Although this initial
      image is coarse, it is useful in many situations.  For example, if
      the image is a World Wide Web imagemap that the user has seen
      before, PNG's first pass is often enough to determine where to
      click.  The PNG scheme also looks better than GIF's, because
      horizontal and vertical resolution never differ by more than a
      factor of two; this avoids the odd "stretched" look seen when
      interlaced GIFs are filled in by replicating scanlines.
      Preliminary results show that small text in an interlaced PNG
      image is typically readable about twice as fast as in an
      equivalent GIF, i.e., after PNG's fifth pass or 25% of the image
      data, instead of after GIF's third pass or 50%.  This is again due
      to PNG's more balanced increase in resolution.

   12.7. Why gamma?

      It might seem natural to standardize on storing sample values
      proportional to display output intensity (that is, have gamma of
      1.0).  But in fact, it is common for images to have a gamma of
      less than 1.  There are three good reasons for this:

          * CRTs have a transfer function with an exponent of 2.2, and
            video signals are designed to be sent directly to CRTs.
            Therefore, images obtained by frame-grabbing video already
            have a gamma of 1/2.2.

          * The human eye has a nonlinear response to intensity, so
            linear encoding of samples either wastes sample codes in
            bright areas of the image, or provides too few sample codes
            to avoid banding artifacts in dark areas of the image, or
            both.  At least 12 bits per sample are needed to avoid
            visible artifacts in linear encoding with a 100:1 image
            intensity range.  An image gamma in the range 0.3 to 0.5
            allocates sample values in a way that roughly corresponds to
            the eye's response, so that 8 bits/sample are enough to
            avoid artifacts caused by insufficient sample precision in
            almost all images.  This makes "gamma encoding" a much
            better way of storing digital images than the simpler linear
            encoding.

          * Many images are created on PCs or workstations with no gamma
            correction hardware and no software willing to provide gamma
            correction either.  In these cases, the images have had
            their lighting and color chosen to look best on this
            platform--they can be thought of as having "manual" gamma
            correction built in.  To see what the image author intended,
            it is necessary to treat such images as having a gamma value
            of 1/2.2 (assuming the author was using a CRT).

      In practice, image gamma values around 1.0, 1/2.2, and 1/1.45 are
      all widely found.  Older image standards such as GIF and JFIF
      often do not account for this fact.  The exchange of images among
      a variety of systems has led to widespread problems with images
      appearing "too dark" or "too light".

      PNG expects viewers to compensate for image gamma at the time that
      the image is displayed.  Another possible approach is to expect
      encoders to convert all images to a uniform gamma at encoding
      time.  While that method would speed viewers slightly, it has
      fundamental flaws:

          * Gamma correction is inherently lossy due to quantization and
            roundoff error.  Requiring conversion at encoding time thus
            causes irreversible loss.  Since PNG is intended to be a
            lossless storage format, this is undesirable; we should
            store unmodified source data.

          * The encoder might not know the source gamma value.  If the
            decoder does gamma correction at viewing time, it can adjust
            the gamma (change the displayed brightness) in response to
            feedback from a human user.  The encoder has no such
            recourse.

          * Whatever "standard" gamma we settled on would be wrong for
            some displays.  Hence viewers would still need gamma
            correction capability.

      Since there will always be images with no gamma or an incorrect
      recorded gamma, good viewers will need to incorporate gamma
      adjustment code anyway.  Gamma correction at viewing time is thus
      the right way to go.

      Historical note:  Version 1.0 of this specification used the gAMA
      chunk to express the relationship between the file samples and the
      "original scene intensity" (camera input) rather than the desired
      display output intensity.  This was changed in version 1.1 for the
      following reasons:

          * The decoder needs to know the desired display output in
            order to do its job, but there was not enough information in
            the file to convert from the original scene to the display
            output.  The version 1.0 specification claimed that the
            conversion depended only on viewing conditions at the
            display, but that was an error; it also depends on
            conditions at the camera.

          * Faithful reproduction of the original scene is not always
            the goal.  Sometimes deliberate distortion is desired.

          * For hand-drawn images there is no "original scene".

      Because the gamma-related recommendations in version 1.0 were
      imprecise, it was not clear what value to put in a gAMA chunk in
      common situations.  For an image drawn on a CRT display with no
      LUT under unknown viewing conditions, an argument could be made
      for any value between 40000 and 50000.  Real applications were
      observed to write 45000 or 45455, and the latter is recommended by
      the current specification.

      See Gamma Tutorial (Chapter 13) for more information.

   12.8. Non-premultiplied alpha

      PNG uses "unassociated" or "non-premultiplied" alpha so that
      images with separate transparency masks can be stored losslessly.
      Another common technique, "premultiplied alpha", stores pixel
      values premultiplied by the alpha fraction; in effect, the image
      is already composited against a black background.  Any image data
      hidden by the transparency mask is irretrievably lost by that
      method, since multiplying by a zero alpha value always produces
      zero.

      Some image rendering techniques generate images with premultiplied
      alpha (the alpha value actually represents how much of the pixel
      is covered by the image).  This representation can be converted to
      PNG by dividing the sample values by alpha, except where alpha is
      zero.  The result will look good if displayed by a viewer that
      handles alpha properly, but will not look very good if the viewer
      ignores the alpha channel.

      Although each form of alpha storage has its advantages, we did not
      want to require all PNG viewers to handle both forms.  We
      standardized on non-premultiplied alpha as being the lossless and
      more general case.

   12.9. Filtering

      PNG includes filtering capability because filtering can
      significantly reduce the compressed size of truecolor and
      grayscale images.  Filtering is also sometimes of value on
      indexed-color images, although this is less common.

      The filter algorithms are defined to operate on bytes, rather than
      pixels; this gains simplicity and speed with very little cost in
      compression performance.  Tests have shown that filtering is
      usually ineffective for images with fewer than 8 bits per sample,
      so providing pixelwise filtering for such images would be
      pointless.  For 16 bit/sample data, bytewise filtering is nearly
      as effective as pixelwise filtering, because MSBs are predicted
      from adjacent MSBs, and LSBs are predicted from adjacent LSBs.

      The encoder is allowed to change filters for each new scanline.
      This creates no additional complexity for decoders, since a
      decoder is required to contain defiltering logic for every filter
      type anyway.  The only cost is an extra byte per scanline in the
      pre-compression datastream.  Our tests showed that when the same
      filter is selected for all scanlines, this extra byte compresses
      away to almost nothing, so there is little storage cost compared
      to a fixed filter specified for the whole image.  And the
      potential benefits of adaptive filtering are too great to ignore.
      Even with the simplistic filter-choice heuristics so far
      discovered, adaptive filtering usually outperforms fixed filters.
      In particular, an adaptive filter can change behavior for
      successive passes of an interlaced image; a fixed filter cannot.

   12.10. Text strings

      Most graphics file formats include the ability to store some
      textual information along with the image.  But many applications
      need more than that: they want to be able to store several
      identifiable pieces of text.  For example, a database using PNG
      files to store medical X-rays would likely want to include
      patient's name, doctor's name, etc.  A simple way to do this in
      PNG would be to invent new private chunks holding text.  The
      disadvantage of such an approach is that other applications would
      have no idea what was in those chunks, and would simply ignore
      them.  Instead, we recommend that textual information be stored in
      standard tEXt chunks with suitable keywords.  Use of tEXt tells
      any PNG viewer that the chunk contains text that might be of
      interest to a human user.  Thus, a person looking at the file with
      another viewer will still be able to see the text, and even
      understand what it is if the keywords are reasonably self-
      explanatory.  (To this end, we recommend spelled-out keywords, not
      abbreviations that will be hard for a person to understand.
      Saving a few bytes on a keyword is false economy.)

      The ISO 8859-1 (Latin-1) character set was chosen as a compromise
      between functionality and portability.  Some platforms cannot
      display anything more than 7-bit ASCII characters, while others
      can handle characters beyond the Latin-1 set.  We felt that
      Latin-1 represents a widely useful and reasonably portable
      character set.  Latin-1 is a direct subset of character sets
      commonly used on popular platforms such as Microsoft Windows and X
      Windows.  It can also be handled on Macintosh systems with a
      simple remapping of characters.

      There is presently no provision for text employing character sets
      other than Latin-1.  We recognize that the need for other
      character sets will increase.  However, PNG already requires that
      programmers implement a number of new and unfamiliar features, and
      text representation is not PNG's primary purpose.  Since PNG
      provides for the creation and public registration of new ancillary
      chunks of general interest, we expect that text chunks for other
      character sets, such as Unicode, eventually will be registered and
      increase gradually in popularity.

   12.11. PNG file signature

      The first eight bytes of a PNG file always contain the following
      values:

         (decimal)              137  80  78  71  13  10  26  10
         (hexadecimal)           89  50  4e  47  0d  0a  1a  0a
         (ASCII C notation)    \211   P   N   G  \r  \n \032 \n

      This signature both identifies the file as a PNG file and provides
      for immediate detection of common file-transfer problems.  The
      first two bytes distinguish PNG files on systems that expect the
      first two bytes to identify the file type uniquely.  The first
      byte is chosen as a non-ASCII value to reduce the probability that
      a text file may be misrecognized as a PNG file; also, it catches
      bad file transfers that clear bit 7.  Bytes two through four name
      the format.  The CR-LF sequence catches bad file transfers that
      alter newline sequences.  The control-Z character stops file
      display under MS-DOS.  The final line feed checks for the inverse
      of the CR-LF translation problem.

      A decoder may further verify that the next eight bytes contain an
      IHDR chunk header with the correct chunk length; this will catch
      bad transfers that drop or alter null (zero) bytes.

      Note that there is no version number in the signature, nor indeed
      anywhere in the file.  This is intentional: the chunk mechanism
      provides a better, more flexible way to handle format extensions,
      as explained in Chunk naming conventions (Section 12.13).

   12.12. Chunk layout

      The chunk design allows decoders to skip unrecognized or
      uninteresting chunks: it is simply necessary to skip the
      appropriate number of bytes, as determined from the length field.

      Limiting chunk length to (2^31)-1 bytes avoids possible problems
      for implementations that cannot conveniently handle 4-byte
      unsigned values.  In practice, chunks will usually be much shorter
      than that anyway.

      A separate CRC is provided for each chunk in order to detect
      badly-transferred images as quickly as possible.  In particular,
      critical data such as the image dimensions can be validated before
      being used.

      The chunk length is excluded from the CRC so that the CRC can be
      calculated as the data is generated; this avoids a second pass
      over the data in cases where the chunk length is not known in
      advance.  Excluding the length from the CRC does not create any
      extra risk of failing to discover file corruption, since if the
      length is wrong, the CRC check will fail: the CRC will be computed
      on the wrong set of bytes and then be tested against the wrong
      value from the file.

   12.13. Chunk naming conventions

      The chunk naming conventions allow safe, flexible extension of the
      PNG format.  This mechanism is much better than a format version
      number, because it works on a feature-by-feature basis rather than
      being an overall indicator.  Decoders can process newer files if
      and only if the files use no unknown critical features (as
      indicated by finding unknown critical chunks).  Unknown ancillary
      chunks can be safely ignored.  We decided against having an
      overall format version number because experience has shown that
      format version numbers hurt portability as much as they help.
      Version numbers tend to be set unnecessarily high, leading to
      older decoders rejecting files that they could have processed
      (this was a serious problem for several years after the GIF89 spec
      came out, for example).  Furthermore, private extensions can be
      made either critical or ancillary, and standard decoders should
      react appropriately; overall version numbers are no help for
      private extensions.

      A hypothetical chunk for vector graphics would be a critical
      chunk, since if ignored, important parts of the intended image
      would be missing.  A chunk carrying the Mandelbrot set coordinates
      for a fractal image would be ancillary, since other applications
      could display the image without understanding what the image
      represents.  In general, a chunk type should be made critical only
      if it is impossible to display a reasonable representation of the
      intended image without interpreting that chunk.

      The public/private property bit ensures that any newly defined
      public chunk type name cannot conflict with proprietary chunks
      that could be in use somewhere.  However, this does not protect
      users of private chunk names from the possibility that someone
      else may use the same chunk name for a different purpose.  It is a
      good idea to put additional identifying information at the start
      of the data for any private chunk type.

      When a PNG file is modified, certain ancillary chunks may need to
      be changed to reflect changes in other chunks.  For example, a
      histogram chunk needs to be changed if the image data changes.  If
      the file editor does not recognize histogram chunks, copying them
      blindly to a new output file is incorrect; such chunks should be
      dropped.  The safe/unsafe property bit allows ancillary chunks to
      be marked appropriately.

      Not all possible modification scenarios are covered by the
      safe/unsafe semantics.  In particular, chunks that are dependent
      on the total file contents are not supported.  (An example of such
      a chunk is an index of IDAT chunk locations within the file:
      adding a comment chunk would inadvertently break the index.)
      Definition of such chunks is discouraged.  If absolutely necessary
      for a particular application, such chunks can be made critical
      chunks, with consequent loss of portability to other applications.
      In general, ancillary chunks can depend on critical chunks but not
      on other ancillary chunks.  It is expected that mutually dependent
      information should be put into a single chunk.

      In some situations it may be unavoidable to make one ancillary
      chunk dependent on another.  Although the chunk property bits are
      insufficient to represent this case, a simple solution is
      available: in the dependent chunk, record the CRC of the chunk
      depended on.  It can then be determined whether that chunk has
      been changed by some other program.

      The same technique can be useful for other purposes.  For example,
      if a program relies on the palette being in a particular order, it
      can store a private chunk containing the CRC of the PLTE chunk.
      If this value matches when the file is again read in, then it
      provides high confidence that the palette has not been tampered
      with.  Note that it is not necessary to mark the private chunk
      unsafe-to-copy when this technique is used; thus, such a private
      chunk can survive other editing of the file.

   12.14. Palette histograms

      A viewer may not be able to provide as many colors as are listed
      in the image's palette.  (For example, some colors could be
      reserved by a window system.)  To produce the best results in this
      situation, it is helpful to have information about the frequency
      with which each palette index actually appears, in order to choose
      the best palette for dithering or to drop the least-used colors.
      Since images are often created once and viewed many times, it
      makes sense to calculate this information in the encoder, although
      it is not mandatory for the encoder to provide it.

      Other image formats have usually addressed this problem by
      specifying that the palette entries should appear in order of
      frequency of use.  That is an inferior solution, because it
      doesn't give the viewer nearly as much information: the viewer
      can't determine how much damage will be done by dropping the last
      few colors.  Nor does a sorted palette give enough information to
      choose a target palette for dithering, in the case that the viewer
      needs to reduce the number of colors substantially.  A palette
      histogram provides the information needed to choose such a target
      palette without making a pass over the image data.

13. Appendix: Gamma Tutorial

   (This appendix is not part of the formal PNG specification.)

   13.1. Nonlinear transfer functions

      It would be convenient for graphics programmers if all of the
      components of an imaging system were linear.  The voltage coming
      from an electronic camera would be directly proportional to the
      intensity (power) of light in the scene, the light emitted by a
      CRT would be directly proportional to its input voltage, and so
      on.  However, real-world devices do not behave in this way.  All
      CRT displays, almost all photographic film, and many electronic
      cameras have nonlinear signal-to-light-intensity or intensity-to-
      signal characteristics.

      Fortunately, all of these nonlinear devices have a transfer
      function that is approximated fairly well by a single type of
      mathematical function: a power function.  This power function has
      the general equation

         output = input ^ exponent

      where ^ denotes exponentiation.  The exponent is often called
      "gamma" and denoted by the Greek letter gamma.

      By convention, input and output are both scaled to the range 0 to
      1, with 0 representing black and 1 representing maximum white (or
      red, etc).  Normalized in this way, the power function is
      completely described by the exponent.

      So, given a particular device, we can measure its output as a
      function of its input, fit a power function to this measured
      transfer function, and extract the exponent.  People often say
      "this device has a gamma of 2.2" as a shorthand for "this device
      has a power-law response with an exponent of 2.2".  People also
      talk about the gamma of a mathematical transform, or of a lookup
      table in a frame buffer, if its input and output are related by
      the power-law expression above.

      But using the term "gamma" to refer to the exponents of transfer
      functions of many different stages in imaging pipelines has led to
      confusion.  Therefore, this specification uses "gamma" to refer
      specifically to the function from display output to image samples,
      and simply uses "exponent" when referring to other functions.

   13.2. Combining exponents

      Real imaging systems will have several components, and more than
      one of these can be nonlinear.  If all of the components have
      transfer characteristics that are power functions, then the
      transfer function of the entire system is also a power function.
      The exponent of the whole system's transfer function is just the
      product of all of the individual exponents of the separate stages
      in the system.

      Also, stages that are linear pose no problem, since a power
      function with an exponent of 1.0 is really a linear function.  So
      a linear transfer function is just a special case of a power
      function, with an exponent of 1.0.

      Thus, as long as our imaging system contains only stages with
      linear and power-law transfer functions, we can meaningfully talk
      about the exponent of the entire system.  This is indeed the case
      with most real imaging systems.

   13.3. End-to-end exponent

      If the end-to-end exponent of an imaging system is 1.0, its output
      is proportional to its input.  This means that the ratio between
      the intensities of any two areas in the reproduced image will be
      the same as it was in the original scene.  It might seem that this
      should always be the goal of an imaging system: to accurately
      reproduce the tones of the original scene.  Alas, that is not the
      case.

      One complication is that the response of the human visual system
      to low light levels is not a scaled-down version of its response
      to high light levels.  Therefore, if the display device emits less
      intense light than entered the capture device (as is usually the
      case for television cameras and television sets, for example), an
      end-to-end linear response will not produce an image that appears
      correct.  There are also other perceptual factors, like the affect
      of the ambient light level and the field of view surrounding the
      display, and physical factors, like reflectance of ambient light
      off the display.

      Good end-to-end exponents are determined from experience.  For
      example, for photographic prints it's about 1.0; for slides
      intended to be projected in a dark room it's about 1.5; for
      television it's about 1.14.

   13.4. CRT exponent

      All CRT displays have a power-law transfer characteristic with an
      exponent of about 2.2.  This is mainly due to the physical
      processes involved in controlling the electron beam in the
      electron gun.

      An exception to this rule is fancy "calibrated" CRTs that have
      internal electronics to alter their transfer function.  If you
      have one of these, you probably should believe what the
      manufacturer tells you its exponent is.  But in all other cases,
      assuming 2.2 is likely to be pretty accurate.

      There are various images around that purport to measure a display
      system's exponent, usually by comparing the intensity of an area
      containing alternating white and black with a series of areas of
      continuous gray of different intensity.  These are usually not
      reliable.  Test images that use a "checkerboard" pattern of black
      and white are the worst, because a single white pixel will be
      reproduced considerably darker than a large area of white.  An
      image that uses alternating black and white horizontal lines (such
      as the gamma.png test image at
      ftp://ftp.uu.net/graphics/png/images/suite/gamma.png) is much
      better, but even it may be inaccurate at high "picture" settings
      on some CRTs.

      If you have a good photometer, you can measure the actual light
      output of a CRT as a function of input voltage and fit a power
      function to the measurements.  However, note that this procedure
      is very sensitive to the CRT's black level adjustment, somewhat
      sensitive to its picture adjustment, and also affected by ambient
      light.  Furthermore, CRTs spread some light from bright areas of
      an image into nearby darker areas; a single bright spot against a
      black background may be seen to have a "halo".  Your measuring
      technique will need to minimize the effects of this.

      Because of the difficulty of measuring the exponent, using either
      test images or measuring equipment, you're usually better off just
      assuming 2.2 rather than trying to measure it.

   13.5. Gamma correction

      A CRT has an exponent of 2.2, and we can't change that.  To get an
      end-to-end exponent closer to 1, we need to have at least one
      other component of the "image pipeline" that is nonlinear.  If, in
      fact, there is only one nonlinear stage in addition to the CRT,
      then it's traditional to say that the other nonlinear stage
      provides "gamma correction" to compensate for the CRT.  However,
      exactly where the "correction" is done depends on circumstance.

      In all broadcast video systems, gamma correction is done in the
      camera.  This choice was made because it was more cost effective
      to place the expensive processing in the small number of capture
      devices (studio television cameras) than in the large number of
      broadcast receivers.  The original NTSC video standard required
      cameras to have a transfer function with an exponent of 1/2.2, or
      about 0.45.  Recently, a more complex two-part transfer function
      has been adopted [SMPTE-170M], but its behavior can be well
      approximated by a power function with an exponent of 0.52.  When
      the resulting image is displayed on a CRT with an exponent of 2.2,
      the end-to-end exponent is about 1.14, which has been found to be
      appropriate for typical television studio conditions and
      television viewing conditions.

      These days, video signals are often digitized and stored in
      computer frame buffers.  The digital image is intended to be sent
      through a CRT, which has exponent 2.2, so the image has a gamma of
      1/2.2.

      Computer rendering programs often produce samples proportional to
      scene intensity.  Suppose the desired end-to-end exponent is near
      1, and the program would like to write its samples directly into
      the frame buffer.  For correct display, the CRT output intensity
      must be nearly proportional to the sample values in the frame
      buffer.  This can be done with a special hardware lookup table
      between the frame buffer and the CRT hardware.  The lookup table
      (often called LUT) is loaded with a mapping that implements a
      power function with an exponent near 1/2.2, providing "gamma
      correction" for the CRT gamma.

      Thus, gamma correction sometimes happens before the frame buffer,
      sometimes after.  As long as images created on a particular
      platform are always displayed on that platform, everything is
      fine.  But when people try to exchange images, differences in
      gamma correction conventions often result in images that seem far
      too bright and washed out, or far too dark and contrasty.

   13.6. Benefits of gamma encoding

      So, is it better to do gamma correction before or after the frame
      buffer?

      In an ideal world, sample values would be stored as floating-point
      numbers, there would be lots of precision, and it wouldn't really
      matter much.  But in reality, we're always trying to store images
      in as few bits as we can.

      If we decide to use samples proportional to intensity, and do the
      gamma correction in the frame buffer LUT, it turns out that we
      need to use at least 12 bits for each of red, green, and blue to
      have enough precision in intensity.  With any less than that, we
      will sometimes see "contour bands" or "Mach bands" in the darker
      areas of the image, where two adjacent sample values are still far
      enough apart in intensity for the difference to be visible.

      However, through an interesting coincidence, the human eye's
      subjective perception of brightness is related to the physical
      stimulation of light intensity in a manner that is very much like
      the power function used for gamma correction.  If we apply gamma
      correction to measured (or calculated) light intensity before
      quantizing to an integer for storage in a frame buffer, we can get
      away with using many fewer bits to store the image.  In fact, 8
      bits per color is almost always sufficient to avoid contouring
      artifacts.  This is because, since gamma correction is so closely
      related to human perception, we are assigning our 256 available
      sample codes to intensity values in a manner that approximates how
      visible those intensity changes are to the eye.  Compared to a
      linearly encoded image, we allocate fewer sample values to
      brighter parts of the tonal range and more sample values to the
      darker portions of the tonal range.

      Thus, for the same apparent image quality, images using gamma-
      encoded sample values need only about two-thirds as many bits of
      storage as images using linearly encoded samples.

   13.7. General gamma handling

      When more than two nonlinear transfer functions are involved in
      the image pipeline, the term "gamma correction" becomes too vague.
      If we consider a pipeline that involves capturing (or calculating)
      an image, storing it in an image file, reading the file, and
      displaying the image on some sort of display screen, there are at
      least 5 places in the pipeline that could have nonlinear transfer
      functions.  Let's give a specific name to each exponent:

      camera exponent
         the exponent of the image sensor

      encoding exponent
         the exponent of any transformation performed by the software
         writing the image file

      decoding exponent
         the exponent of any transformation performed by the software
         reading the image file

      LUT exponent
         the exponent of the frame buffer LUT, if present

      CRT exponent
         the exponent of the CRT, generally 2.2

      In addition, let's add a few other names:

      display exponent
         the exponent of the "display system" downstream of the frame
         buffer

            display_exponent = LUT_exponent * CRT_exponent

      gamma
         the exponent of the function mapping display output intensity
         to file samples

            gamma = 1.0 / (decoding_exponent * display_exponent)

      end-to-end exponent
         the exponent of the function mapping image sensor input
         intensity to display output intensity, generally 1.0 to 1.5

      When displaying an image file, the image decoding program is
      responsible for making gamma equal to the value specified in the
      gAMA chunk, by selecting the decoding exponent appropriately:

         decoding_exponent = 1.0 / (gamma * display_exponent)

      The display exponent might be known for this machine, or it might
      be obtained from the system software, or the user might have to be
      asked what it is.

   13.8. Some specific examples

      In digital video systems, the camera exponent is about 0.52 by
      declaration of the various video standards documents.  The CRT
      exponent is 2.2 as usual, while the encoding exponent, decoding
      exponent, and LUT exponent are all 1.0.  As a result, the end-to-
      end exponent is about 1.14.

      On frame buffers that have hardware gamma correction tables, and
      that are calibrated to display samples that are proportional to
      display output intensity, the display exponent is 1.0.

      Many workstations and X terminals and PC clones lack gamma
      correction lookup tables.  Here, the LUT exponent is always 1.0,
      so the display exponent is 2.2.

      On the Macintosh, there is a LUT.  By default, it is loaded with a
      table whose exponent is 1/1.45, giving a display exponent (LUT and
      CRT combined) of about 1.52.  Some Macs have a "Gamma" control
      panel with selections labeled 1.0, 1.2, 1.4, 1.8, or 2.2.  These
      settings load alternate LUTs, but beware: the selection labeled
      with the value g loads a LUT with exponent g/2.61, yielding

         display_exponent = (g/2.61) * 2.2

      On recent SGI systems, there is a hardware gamma-correction table
      whose contents are controlled by the (privileged) gamma program.
      The exponent of the table is actually the reciprocal of the number
      g that gamma prints.  You can obtain g from the file
      /etc/config/system.glGammaVal and calculate

         display_exponent = 2.2 / g

      You will find SGI systems with g set to 1.0 and 2.2 (or higher),
      but the default when machines are shipped is 1.7.

      On NeXT systems the LUT has exponent 1/2.2 by default, but it can
      be modified by third-party applications.

      In summary, for images designed to need no correction on these
      platforms:

         Platform    LUT exponent   Default LUT exponent   Default gAMA

         PC clone       1.0                     1.0           45455
         Macintosh     g/2.61        1.8/2.61 = 1/1.45        65909
         SGI            1/g                     1/1.7         77273
         NeXT           1/g                     1/2.2        100000

      The default gAMA values assume a CRT display.

   13.9. Video camera transfer functions

      The original NTSC video standards specified a simple power-law
      camera transfer function with an exponent of 1/2.2 (about 0.45).
      This is not possible to implement exactly in analog hardware
      because the function has infinite slope at x=0, so all cameras
      deviated to some degree from this ideal.  More recently, a new
      camera transfer function that is physically realizable has been
      accepted as a standard [SMPTE-170M]. It is

         if (Vin < 0.018)  Vout = 4.5 * Vin
         if (Vin >= 0.018) Vout = 1.099 * (Vin^0.45) - 0.099

      where Vin and Vout are measured on a scale of 0 to 1.  Although
      the exponent remains 0.45, the multiplication and subtraction
      change the shape of the transfer function, so it is no longer a
      pure power function.  It can be well approximated, however, by a
      power function with exponent 0.52.

      The PAL and SECAM video standards specify a power-law camera
      transfer function with an exponent of 1/2.8 (about 0.36).
      However, this is too low in practice, so real cameras are likely
      to have exponents close to NTSC practice.

   13.10. Further reading

      Charles Poynton's "Gamma FAQ" [GAMMA-FAQ] is a excellent source of
      information about gamma, although it claims that CRTs have an
      exponent of 2.5.  See also his book [DIGITAL-VIDEO].

14. Appendix: Color Tutorial

   (This appendix is not part of the formal PNG specification.)

   14.1. About chromaticity

      The cHRM chunk is used, together with the gAMA chunk, to convey
      precise color information so that a PNG image can be displayed or
      printed with better color fidelity than is possible without this
      information.  The preceding chapters state how this information is
      encoded in a PNG image.  This tutorial briefly outlines the
      underlying color theory for those who might not be familiar with
      it.

      Note that displaying an image with incorrect gamma will produce
      much larger color errors than failing to use the chromaticity
      data.  First be sure the monitor set-up and gamma correction are
      right, then worry about chromaticity.

   14.2. The problem of color

      The color of an object depends not only on the precise spectrum of
      light emitted or reflected from it, but also on the observer--
      their species, what else they can see at the same time, even what
      they have recently looked at!  Furthermore, two very different
      spectra can produce exactly the same color sensation.  Color is
      not an objective property of real-world objects; it is a
      subjective, biological sensation.  However, by making some
      simplifying assumptions (such as: we are talking about human
      vision) it is possible to produce a mathematical model of color
      and thereby obtain good color accuracy.

   14.3. Device-dependent color

      Display the same RGB data on three different monitors, side by
      side, and you will get a noticeably different color balance on
      each display.  This is because each monitor emits a slightly
      different shade and intensity of red, green, and blue light.  RGB
      is an example of a device-dependent color model--the color you get
      depends on the device.  This also means that a particular color--
      represented as say RGB 87, 146, 116 on one monitor--might have to
      be specified as RGB 98, 123, 104 on another to produce the same
      color.

   14.4. Device-independent color

      A full physical description of a color would require specifying
      the exact spectral power distribution of the light source.
      Fortunately, the human eye and brain are not so sensitive as to
      require exact reproduction of a spectrum.  Mathematical, device-
      independent color models exist that describe fairly well how a
      particular color will be seen by humans.  The most important
      device-independent color model, to which all others can be
      related, was developed by the International Commission on
      Illumination (CIE, in French) and is called "CIE XYZ" or simply
      "XYZ".

      In XYZ, X is the sum of a weighted power distribution over the
      whole visible spectrum.  So are Y and Z, each with different
      weights.  Thus any arbitrary spectral power distribution is
      condensed down to just three floating-point numbers.  The weights
      were derived from color matching experiments done on human
      subjects in the 1920s.  CIE XYZ has been an International Standard
      since 1931, and it has a number of useful properties:

          * two colors with the same XYZ values will look the same to
            humans

          * two colors with different XYZ values will not look the same

          * the Y value represents all the brightness information
            (luminance)

          * the XYZ color of any object can be objectively measured

      Color models based on XYZ have been used for many years by people
      who need accurate control of color--lighting engineers for film
      and TV, paint and dyestuffs manufacturers, and so on.  They are
      thus proven in industrial use.  Accurate, device-independent color
      started to spread from high-end, specialized areas into the
      mainstream during the late 1980s and early 1990s, and PNG takes
      notice of that trend.

   14.5. Calibrated device-dependent color

      Traditionally, image file formats have used uncalibrated, device-
      dependent color.  If the precise details of the original display
      device are known, it becomes possible to convert the device-
      dependent colors of a particular image to device-independent ones.
      Making simplifying assumptions, such as working with CRTs (which
      are much easier than printers), all we need to know are the XYZ
      values of each primary color and the CRT exponent.

      So why does PNG not store images in XYZ instead of RGB?  Well, two
      reasons.  First, storing images in XYZ would require more bits of
      precision, which would make the files bigger.  Second, all
      programs would have to convert the image data before viewing it.
      Whether calibrated or not, all variants of RGB are close enough
      that undemanding viewers can get by with simply displaying the
      data without color correction.  By storing calibrated RGB, PNG
      retains compatibility with existing programs that expect RGB data,
      yet provides enough information for conversion to XYZ in
      applications that need precise colors.  Thus, we get the best of
      both worlds.

   14.6. Chromaticity and luminance

      Chromaticity is an objective measurement of the color of an
      object, leaving aside the brightness information.  Chromaticity
      uses two parameters x and y, which are readily calculated from
      XYZ:

         x = X / (X + Y + Z)
         y = Y / (X + Y + Z)

      XYZ colors having the same chromaticity values will appear to have
      the same hue but can vary in absolute brightness.  Notice that x,y
      are dimensionless ratios, so they have the same values no matter
      what units we've used for X,Y,Z.

      The Y value of an XYZ color is directly proportional to its
      absolute brightness and is called the luminance of the color.  We
      can describe a color either by XYZ coordinates or by chromaticity
      x,y plus luminance Y.  The XYZ form has the advantage that it is
      linearly related to RGB intensities.

   14.7. Characterizing computer monitors

      The "white point" of a monitor is the chromaticity x,y of the
      monitor's nominal white, that is, the color produced when
      R=G=B=maximum.

      It's customary to specify monitor colors by giving the
      chromaticities of the individual phosphors R, G, and B, plus the
      white point.  The white point allows one to infer the relative
      brightnesses of the three phosphors, which isn't determined by
      their chromaticities alone.

      Note that the absolute brightness of the monitor is not specified.
      For computer graphics work, we generally don't care very much
      about absolute brightness levels.  Instead of dealing with
      absolute XYZ values (in which X,Y,Z are expressed in physical
      units of radiated power, such as candelas per square meter), it is
      convenient to work in "relative XYZ" units, where the monitor's
      nominal white is taken to have a luminance (Y) of 1.0.  Given this
      assumption, it's simple to compute XYZ coordinates for the
      monitor's white, red, green, and blue from their chromaticity
      values.

      Why does cHRM use x,y rather than XYZ?  Simply because that is how
      manufacturers print the information in their spec sheets!
      Usually, the first thing a program will do is convert the cHRM
      chromaticities into relative XYZ space.

   14.8. Uses for XYZ

      If a PNG file has the gAMA and cHRM chunks, the source RGB values
      can be converted to XYZ.  This lets you:

          * do accurate grayscale conversion (just use the Y component)

          * convert to RGB for your own monitor (to see the original
            colors)

          * print the image in Level 2 PostScript with better color
            fidelity than a simple RGB to CMYK conversion could provide

          * calculate an optimal color palette

          * pass the image data to a color management system

          * etc.

   14.9. Converting between RGB and XYZ

      Make a few simplifying assumptions first, like the monitor really
      is jet black with no input and the guns don't interfere with one
      another.  Then, given that you know the CIE XYZ values for each of
      red, green, and blue for a particular monitor, you put them into a
      matrix M:

              Xr Xg Xb
         M =  Yr Yg Yb
              Zr Zg Zb

      RGB intensity samples normalized to the range 0 to 1 can be
      converted to XYZ by matrix multiplication.  (If you have gamma-
      encoded RGB samples, first undo the gamma encoding.)

         X     R
         Y = M G
         Z     B

      In other words, X = Xr*R + Xg*G + Xb*B, and similarly for Y and Z.
      You can go the other way too:

         R        X
         G = invM Y
         B        Z

      where invM is the inverse of the matrix M.

   14.10. Device gamut

      The gamut of a device is the subset of visible colors that the
      device can display.  (It has nothing to do with gamma.)  The gamut
      of an RGB device can be visualized as a polyhedron in XYZ space;
      the vertices correspond to the device's black, blue, red, green,
      magenta, cyan, yellow, and white.

      Different devices have different gamuts, in other words one device
      will be able to display certain colors (usually highly saturated
      ones) that another device cannot.  The gamut of a particular RGB
      device can be determined from its R, G, and B chromaticities and
      white point (the same values given in the cHRM chunk).  The gamut
      of a color printer is more complex and can be determined only by
      measurement.  However, printer gamuts are typically smaller than
      monitor gamuts, meaning that there can be many colors in a
      displayable image that cannot physically be printed.

      Converting image data from one device to another generally results
      in gamut mismatches--colors that cannot be represented exactly on
      the destination device.  The process of making the colors fit,
      which can range from a simple clip to elaborate nonlinear scaling
      transformations, is termed gamut mapping.  The aim is to produce a
      reasonable visual representation of the original image.

   Further reading

   14.11. Further reading

      References [COLOR-1] through [COLOR-5] provide more detail about
      color theory.

15. Appendix: Sample CRC Code

   The following sample code represents a practical implementation of
   the CRC (Cyclic Redundancy Check) employed in PNG chunks.  (See also
   ISO 3309 [ISO-3309] or ITU-T V.42 [ITU-T-V42] for a formal
   specification.)

   The sample code is in the ANSI C programming language.  Non C users
   may find it easier to read with these hints:

   &
      Bitwise AND operator.

   ^
      Bitwise exclusive-OR operator.  (Caution: elsewhere in this
      document, ^ represents exponentiation.)

   >>
      Bitwise right shift operator.  When applied to an unsigned
      quantity, as here, right shift inserts zeroes at the left.

   !
      Logical NOT operator.

   ++
      N++ increments the variable N.

   0xNNN
      0x introduces a hexadecimal (base 16) constant.  Suffix L
      indicates a long value (at least 32 bits).

      /* Table of CRCs of all 8-bit messages. */
      unsigned long crc_table[256];

      /* Flag: has the table been computed? Initially false. */
      int crc_table_computed = 0;

      /* Make the table for a fast CRC. */
      void make_crc_table(void)
      {
        unsigned long c;
        int n, k;

        for (n = 0; n < 256; n++) {
          c = (unsigned long) n;
          for (k = 0; k < 8; k++) {
            if (c & 1)
              c = 0xedb88320L ^ (c >> 1);
            else
              c = c >> 1;
          }
          crc_table[n] = c;
        }
        crc_table_computed = 1;
      }

      /* Update a running CRC with the bytes buf[0..len-1]--the CRC
         should be initialized to all 1's, and the transmitted value
         is the 1's complement of the final running CRC (see the
         crc() routine below)). */

      unsigned long update_crc(unsigned long crc, unsigned char *buf,
                               int len)
      {
        unsigned long c = crc;
        int n;

        if (!crc_table_computed)
          make_crc_table();
        for (n = 0; n < len; n++) {
          c = crc_table[(c ^ buf[n]) & 0xff] ^ (c >> 8);
        }
        return c;
      }

      /* Return the CRC of the bytes buf[0..len-1]. */
      unsigned long crc(unsigned char *buf, int len)
      {
        return update_crc(0xffffffffL, buf, len) ^ 0xffffffffL;
      }

16. Appendix: Online Resources

   (This appendix is not part of the formal PNG specification.)

   This appendix gives the locations of some Internet resources for PNG
   software developers.  By the nature of the Internet, the list is
   incomplete and subject to change.

   Archive sites

      The latest released versions of this document and related
      information can always be found at the PNG FTP archive site,
      ftp://ftp.uu.net/graphics/png/. The PNG specification is available
      in several formats, including HTML, plain text, and PostScript.

   Reference implementation and test images

      A reference implementation in portable C is available from the PNG
      FTP archive site, ftp://ftp.uu.net/graphics/png/src/. The
      reference implementation is freely usable in all applications,
      including commercial applications.

      Test images are available from
      ftp://ftp.uu.net/graphics/png/images/.

   Electronic mail

      The maintainers of the PNG specification can be contacted by
      e-mail at [e-mail] or at [e-mail].

   PNG web site

      There is a World Wide Web site for PNG at
      http://www.cdrom.com/pub/png/. This is a central location for
      current information about PNG and PNG-related tools.

17. Appendix: Revision History

   (This appendix is not part of the formal PNG specification.)

   The PNG format has been frozen since the Ninth Draft of 7 March 1995,
   and all future changes are intended to be backward compatible.  The
   revisions since the Ninth Draft are simply clarifications,
   improvements in presentation, additions of supporting material, and
   specifications for additional chunks.

   On 1 October 1996, the PNG 1.0 specification was approved as a W3C
   (World Wide Web Consortium) Recommendation.

   In January 1997 it was published as RFC-2083 (informational) with
   technical content identical to that of the W3C Recommendation.

   In October 1998, the PNG 1.1 specification was approved by the PNG
   Development Group.

   Changes since version 1.0

      (W3C Recommendation 01-October-1996, RFC 2083 January 1997)

      Note: These changes have been approved by the PNG Development
      Group, but not by any standards body.

          * Redefined gAMA to be in terms of the desired display output
            rather than the original scene, and revised all discussions
            of gamma and references to gamma accordingly

          * Added the iCCP, sPLT, and sRGB chunks

          * Extended the scope of the 31-bit limit on chunk lengths and
            image dimensions to apply to all four-byte unsigned
            integers, and disallowed the value -2^31 in four-byte signed
            integers

          * Mentioned the possibility of dithering the alpha channel
            when converting it to binary transparency

          * Clarified that zlib window sizes smaller than 32K are valid

          * Updated the PNG web site URL and authors' email addresses

          * Editing and reformatting

   Changes since the Tenth Draft of 5 May 1995

          * Clarified meaning of a suggested-palette PLTE chunk in a
            truecolor image that uses transparency

          * Clarified exact semantics of sBIT and allowed sample depth
            scaling procedures

          * Clarified status of spaces in tEXt chunk keywords

          * Distinguished private and public extension values in type
            and method fields

          * Added a "Creation Time" tEXt keyword

          * Macintosh representation of PNG specified

          * Added discussion of security issues

          * Added more extensive discussion of gamma and chromaticity
            handling, including tutorial appendixes

          * Clarified terminology, notably sample depth vs. bit depth

          * Added a glossary

          * Editing and reformatting

18. References

   [COLOR-1]
      Hall, Roy, Illumination and Color in Computer Generated Imagery.
      Springer-Verlag, New York, 1989.  ISBN 0-387-96774-5.

   [COLOR-2]
      Kasson, J., and W. Plouffe, "An Analysis of Selected Computer
      Interchange Color Spaces", ACM Transactions on Graphics, vol 11 no
      4 (1992), pp 373-405.

   [COLOR-3]
      Lilley, C., F. Lin, W.T. Hewitt, and T.L.J. Howard, Colour in
      Computer Graphics. CVCP, Sheffield, 1993.  ISBN 1-85889-022-5.

   [COLOR-4]
      Stone, M.C., W.B. Cowan, and J.C. Beatty, "Color gamut mapping and
      the printing of digital images", ACM Transactions on Graphics, vol
      7 no 3 (1988), pp 249-292.

   [COLOR-5]
      Travis, David, Effective Color Displays--Theory and Practice.
      Academic Press, London, 1991.  ISBN 0-12-697690-2.

   [DIGITAL-VIDEO]
      Poynton, Charles, A Technical Introduction to Digital Video.  John
      Wiley & Sons, 1996.  ISBN 0-471-12253-X.
      http://www.inforamp.net/~poynton/Poynton-T-I-Digital-Video.html

   [GAMMA-FAQ]
      Poynton, C., "Gamma FAQ".
      http://www.inforamp.net/~poynton/Poynton-colour.html

   [ICC]
      The International Color Consortium
      http://www.color.org/

   [ISO-3309]
      International Organization for Standardization, "Information
      Processing Systems--Data Communication High-Level Data Link
      Control Procedure--Frame Structure", IS 3309, October 1984, 3rd
      Edition.

   [ISO-3664]
      International Organization for Standardization, "Photography--
      Illumination Conditions for Viewing Colour Transparencies and
      their Representation", IS 3664, 1975.

   [ISO/IEC-8859-1]
      International Organization for Standardization and International
      Electrotechnical Commission, "Information Technology--8-bit
      Single-Byte Coded Graphic Character Sets--Part 1: Latin Alphabet
      No. 1", IS 8859-1, 1998.
      Also see sample files at
      ftp://ftp.uu.net/graphics/png/documents/iso_8859-1.*

   [ITU-R-BT709]
      International Telecommunications Union, "Basic Parameter Values
      for the HDTV Standard for the Studio and for International
      Programme Exchange", ITU-R Recommendation BT.709 (formerly CCIR
      Rec. 709), 1990.

   [ITU-T-V42]
      International Telecommunications Union, "Error-correcting
      Procedures for DCEs Using Asynchronous-to-Synchronous Conversion",
      ITU-T Recommendation V.42, 1994, Rev. 1.

   [PAETH]
      Paeth, A.W., "Image File Compression Made Easy", in Graphics Gems
      II, James Arvo, editor.  Academic Press, San Diego, 1991.  ISBN
      0-12-064480-0.

   [POSTSCRIPT]
      Adobe Systems Incorporated, PostScript Language Reference Manual,
      2nd edition.  Addison-Wesley, Reading, 1990.  ISBN 0-201-18127-4.

   [PNG-EXTENSIONS]
      PNG Group, "Extensions to the PNG 1.1 Specification, Version
      1.1.0".  Available in several formats from
      ftp://ftp.uu.net/graphics/png/documents/pngext-*

   [RFC-1123]
      Braden, R., Editor, "Requirements for Internet Hosts--Application
      and Support", STD 3, RFC 1123, USC/Information Sciences Institute,
      October 1989.
      ftp://ftp.isi.edu/in-notes/rfc1123.txt

   [RFC-1950]
      Deutsch, P. and J-L. Gailly, "ZLIB Compressed Data Format
      Specification version 3.3", RFC 1950, Aladdin Enterprises,
      May 1996.
      ftp://ftp.isi.edu/in-notes/rfc1950.txt

   [RFC-1951]
      Deutsch, P., "DEFLATE Compressed Data Format Specification version
      1.3", RFC 1951, Aladdin Enterprises, May 1996.
      ftp://ftp.isi.edu/in-notes/rfc1951.txt

   [RFC-2045]
      Freed, N., and N. Borenstein, "Multipurpose Internet Mail
      Extensions (MIME) Part One: Format of Internet Message Bodies",
      RFC 2045, Innosoft, First Virtual, November 1996.
      ftp://ftp.isi.edu/in-notes/rfc2045.txt

   [RFC-2048]
      Freed, N., Klensin, J., and J. Postel, "Multipurpose Internet Mail
      Extensions (MIME) Part Four: Registration Procedures", RFC 2048,
      Innosoft, MCI, USC/Information Sciences Institute, November 1996.
      ftp://ftp.isi.edu/in-notes/rfc2048.txt

   [RFC-2119]
      Bradner, Scott, "Key words for use in RFCs to Indicate Requirement
      Levels", RFC 2119, Harvard University, March 1997.
      ftp://ftp.isi.edu/in-notes/rfc2119.txt

   [SMPTE-170M]
      Society of Motion Picture and Television Engineers, "Television--
      Composite Analog Video Signal--NTSC for Studio Applications",
      SMPTE-170M, 1994.

   [sRGB]
      Stokes, Michael, Matthew Anderson, Srinivasan Chandrasekar, and
      Ricardo Motta, A Standard Default Color Space for the Internet -
      sRGB.
      http://www.w3.org/Graphics/Color/sRGB

      The key portions of this document are being adopted with revisions
      into:

      International Electrotechnical Commission, "Colour Measurement and
      Management in Multimedia Systems and Equipment - Part 2-1: Default
      RGB Colour Space - sRGB", IEC 61966-2-1.
      http://w3.hike.te.chiba-u.ac.jp/IEC/100/PT61966/parts/part2/

19. Credits

   Editors

          * Glenn Randers-Pehrson (PNG 1.1), randeg @ alumni.rpi.edu
          * Thomas Boutell (PNG 1.0), boutell @ boutell.com

   Contributing Editors

          * Adam M. Costello (PNG 1.1), amc @ cs.berkeley.edu
          * Tom Lane (PNG 1.0), tgl @ sss.pgh.pa.us

   Authors

      Authors' names are presented in alphabetical order.  Spaces have
      been added to e-mail addresses (around the @ symbols) to minimize
      the amount of unwanted e-mail via automated harvesters.

          * Mark Adler, madler @ alumni.caltech.edu
          * Thomas Boutell, boutell @ boutell.com
          * John Bowler, jbowler @ acm.org
          * Christian Brunschen, cb @ df.lth.se
          * Adam M. Costello, amc @ cs.berkeley.edu
          * Lee Daniel Crocker, lee @ piclab.com
          * Andreas Dilger, adilger @ enel.ucalgary.ca
          * Oliver Fromme, oliver @ fromme.com
          * Jean-loup Gailly, jloup @ gzip.org
          * Chris Herborth, chrish @ qnx.com
          * Alex Jakulin, Aleks.Jakulin @ snet.fri.uni-lj.si
          * Neal Kettler, neal @ westwood.com
          * Tom Lane, tgl @ sss.pgh.pa.us
          * Alexander Lehmann, alex @ hal.rhein-main.de
          * Chris Lilley, chris @ w3.org
          * Dave Martindale, davem @ cs.ubc.ca
          * Owen Mortensen, ojm @ acm.org
          * Keith S. Pickens, ksp @ swri.edu
          * Robert P. Poole
          * Glenn Randers-Pehrson, randeg @ alumni.rpi.edu
          * Greg Roelofs, newt @ pobox.com
          * Willem van Schaik, willem @ schaik.com
          * Guy Schalnat, gschal @ infinet.com
          * Paul Schmidt, pschmidt @ photodex.com
          * Michael Stokes, Michael_Stokes @ hp.com
          * Tim Wegner, twegner @ phoenix.net
          * Jeremy Wohl, jeremyw @ evantide.com

      The authors wish to acknowledge the contributions of the Portable
      Network Graphics mailing list, the readers of comp.graphics, and
      the members of the World Wide Web Consortium (W3C).

      The Adam7 interlacing scheme is not patented and it is not the
      intention of the originator of that scheme to patent it.  The
      scheme may be freely used by all PNG implementations.  The name
      "Adam7" may be freely used to describe interlace method 1 of the
      PNG specification.

   Trademarks

      GIF is a service mark of CompuServe Incorporated.  IBM PC is a
      trademark of International Business Machines Corporation.
      Macintosh is a trademark of Apple Computer, Inc.  Microsoft,
      Windows, and MS-DOS are trademarks of Microsoft Corporation.
      PhotoCD is a trademark of Eastman Kodak Company.  PostScript and
      TIFF are trademarks of Adobe Systems Incorporated.  SGI is a
      trademark of Silicon Graphics, Inc.  X Window System is a
      trademark of the Massachusetts Institute of Technology.

   Document source

      This document was built from the file png-master-19990130 on
      31 January 1999.

   Copyright notice

      Copyright (c) 1998, 1999 by:  Glenn Randers-Pehrson

      This specification is a modification of the PNG 1.0 specification.
      It is being provided by the copyright holder under the provisions
      of the 1996 MIT copyright and license:

      Copyright (c) 1996 by:  Massachusetts Institute of Technology
      (MIT)

      This W3C specification is being provided by the copyright holders
      under the following license.  By obtaining, using and/or copying
      this specification, you agree that you have read, understood, and
      will comply with the following terms and conditions:

      Permission to use, copy, and distribute this specification for any
      purpose and without fee or royalty is hereby granted, provided
      that the full text of this NOTICE appears on ALL copies of the
      specification or portions thereof, including modifications, that
      you make.

      THIS SPECIFICATION IS PROVIDED "AS IS," AND COPYRIGHT HOLDERS MAKE
      NO REPRESENTATIONS OR WARRANTIES, EXPRESS OR IMPLIED.  BY WAY OF
      EXAMPLE, BUT NOT LIMITATION, COPYRIGHT HOLDERS MAKE NO
      REPRESENTATIONS OR WARRANTIES OF MERCHANTABILITY OR FITNESS FOR
      ANY PARTICULAR PURPOSE OR THAT THE USE OF THE SPECIFICATION WILL
      NOT INFRINGE ANY THIRD PARTY PATENTS, COPYRIGHTS, TRADEMARKS OR
      OTHER RIGHTS.  COPYRIGHT HOLDERS WILL BEAR NO LIABILITY FOR ANY
      USE OF THIS SPECIFICATION.

      The name and trademarks of copyright holders may NOT be used in
      advertising or publicity pertaining to the specification without
      specific, written prior permission.  Title to copyright in this
      specification and any associated documentation will at all times
      remain with copyright holders.

End of PNG Specification