Honeybee: Hot Shareware/Honeybee Hot Shareware (Power Source, Inc.).iso/viewer/gif.txt

Text File | 1992-08-06 | 27KB | 570 lines

                               G I F (tm)
                    Graphics Interchange Format (tm)

          A standard defining a mechanism for the storage and
           transmission of raster-based graphics information

            June 15, 1987 (c) CompuServe Incorporated, 1987
                          All rights reserved

          While this document is copyrighted, the information
         contained within is made available for use in computer
         software without royalties, or licensing restrictions.
        GIF and 'Graphics Interchange Format' are trademarks of
                       CompuServe, Incorporated.
                          an H&R Block Company
                      5000 Arlington Centre Blvd.
                          Columbus, Ohio 43220
                             [phone]

                              INTRODUCTION

     'GIF' (tm) is CompuServe's standard for defining generalized
color raster images.  This 'Graphics Interchange Format' (tm) allows
high-quality, high-resolution graphics to be displayed on a variety of
graphics hardware and is intended as an exchange and display mechanism
for graphics images.  The image format described in this document is
designed to support current and future image technology and will in
addition serve as a basis for future CompuServe graphics products.
     The main focus of this  document is to provide the technical
information necessary for a programmer to implement GIF encoders and
decoders.  As such, some assumptions are made as to terminology
relavent to graphics and programming in general.
     The first section of this document describes the GIF  data  format
and its components and applies to all GIF decoders, either as
standalone programs or as part of a communications  package.   Appendix
B is a section relavent to decoders that are part of a communications
software package and describes the protocol requirements for entering
and exiting GIF mode, and responding to host interrogations.  A
glossary in Appendix A defines some of the terminology used in this
document.  Appendix C gives a detailed explanation of how the graphics
image itself is packaged as a series of data bytes.

Graphics Interchange Format Data Definition

GENERAL FILE FORMAT

+-----------------------+
| +-------------------+ |
| |   GIF Signature   | |
| +-------------------+ |
| +-------------------+ |
| | Screen Descriptor | |
| +-------------------+ |
| +-------------------+ |
| | Global Color Map  | |
| +-------------------+ |
. . .               . . .
| +-------------------+ |    ---+
| |  Image Descriptor | |       |
| +-------------------+ |       |
| +-------------------+ |       |
| |  Local Color Map  | |       |-Repeated 1 to n times
| +-------------------+ |       |
| +-------------------+ |       |
| |    Raster Data    | |       |
| +-------------------+ |    ---+
. . .               . . .
|-    GIF Terminator   -|
+-----------------------+

GIF SIGNATURE

     The following GIF Signature identifies the data following as a
valid GIF image stream.  It consists of the following six characters:

   G I F 8 7 a

     The last three characters '87a' may be viewed as a version number
for this particular GIF definition and will be used in general as a
reference in documents regarding GIF that address any version
dependencies.

SCREEN DESCRIPTOR

     The Screen Descriptor describes the overall parameters for all GIF
images following.  It defines the overall dimensions of the image space
or logical screen required, the existance of color mapping information,
background screen color, and color depth information.  This information
is stored in a series of 8-bit bytes as described below.

bits 7 6 5 4 3 2 1 0 Byte #
+---------------+
|               |  1
+-Screen Width -+      Raster width in pixels (LSB first)
|               |  2
+---------------+
|               |  3
+-Screen Height-+      Raster height in pixels (LSB first)
|               |  4
+-+-----+-+-----+      M = 1, Global color map follows Descriptor
|M|  cr |0|pixel|  5   cr+1 = # bits of color resolution
+-+-----+-+-----+      pixel+1 = # bits/pixel in image
|  background   |  6   background=Color index of screen background
+---------------+        (color is defined from the Global color
|0 0 0 0 0 0 0 0|  7     map or default map if none specified)
+---------------+

     The logical screen width and height can both be larger than the
physical display.  How images larger than the physical display are
handled is implementation dependent and can take advantage of hardware
characteristics (e.g. Macintosh scrolling windows).  Otherwise images
can be clipped to the edges of the display.  The value of 'pixel' also
defines the maximum number of colors within an image.  The range of
values for 'pixel' is 0 to 7 which represents 1 to 8 bits.  This
translates to a range of 2 (B & W) to 256 colors.  Bit 3 of word 5 is
reserved for future definition and must be zero.

GLOBAL COLOR MAP

     The Global Color Map is optional but recommended for images where
accurate color rendition is desired.  The existence of this color map
is indicated in the 'M' field of byte 5 of the Screen Descriptor.  A
color map can also be associated with each image in a GIF file as
described later.  However this global map will normally be used because
of hardware restrictions in equipment available today.  In the
individual Image Descriptors the 'M' flag will normally be zero.  If
the Global Color Map is present, it's definition immediately follows
the Screen Descriptor.   The number of color map entries following a
Screen Descriptor is equal to 2**(# bits per pixel), where each entry
consists of three byte values representing the relative intensities of
red, green and blue respectively.  The structure of the Color Map block
is:

      bits
 7 6 5 4 3 2 1 0  Byte #
+---------------+
| red intensity |  1    Red value for color index 0
+---------------+
|green intensity|  2    Green value for color index 0
+---------------+
| blue intensity|  3    Blue value for color index 0
+---------------+
| red intensity |  4    Red value for color index 1
+---------------+
|green intensity|  5    Green value for color index 1
+---------------+
| blue intensity|  6    Blue value for color index 1
+---------------+
:               :       (Continues for remaining colors)

     Each image pixel value received will be displayed according to
its closest match with an available color of the display based on this
color map.  The color components represent a fractional intensity
value from none (0) to full (255).  White would be represented as
(255,255,255), black as (0,0,0) and medium yellow as (180,180,0).  For
display, if the device supports fewer than 8 bits per color component,
the higher order bits of each component are used.  In the creation of a
GIF color map entry with hardware supporting fewer than 8 bits per
component, the component values for the hardware should be converted to
the 8-bit format with the following calculation:

     <map_value> = <component_value>*255/(2**<nbits> -1)

     This assures accurate translation of colors for all displays.  In
the cases of creating GIF images from hardware without color palette
capability, a fixed palette should be created based on the available
display colors for that hardware.  If no Global Color Map is indicated,
a default color map is generated internally which maps each possible
incoming color index to the same hardware color index modulo <n> where
<n> is the number of available hardware colors.

IMAGE DESCRIPTOR

     The Image Descriptor defines the actual placement and extents of
the following image within the space defined in the Screen Descriptor.
Also defined are flags to indicate the presence of a local color lookup
map, and to define the pixel display sequence.  Each Image Descriptor
is introduced by an image separator  character.   The role of the Image
Separator is simply to provide a synchronization character to introduce
an Image Descriptor.  This is desirable if a GIF file happens to
contain more than one image.   This character is defined as 0x2C hex or
',' (comma).  When this character is encountered between images, the
Image Descriptor will follow immediately.
     Any characters encountered between the end of a previous image and
the image separator character are to be ignored.  This allows future
GIF enhancements to be present in newer image formats and yet ignored
safely by older software decoders.

      bits
 7 6 5 4 3 2 1 0  Byte #
+---------------+
|0 0 1 0 1 1 0 0|  1 ',' -Image separator character
+---------------+
|               |  2    Start of image in pixels from the
+- Image Left  -+       left side of the screen (LSB first)
|               |  3
+---------------+
|               |  4
+-  Image Top  -+       Start of image in pixels from the
|               |  5    top of the screen (LSB first)
+---------------+
|               |  6
+- Image Width -+       Width of the image in pixels (LSB first)
|               |  7
+---------------+
|               |  8
+-Image Height-+        Height of the image in pixels (LSB first)
|               |  9
+-+-+-+-+-+-----+       M=0 -Use global color map, ignore 'pixel'
|M|I|0|0|0|pixel| 10    M=1 - Local color map follows, use 'pixel'
+-+-+-+-+-+-----+       I=0 - Image formatted in Sequential order
                        I=1 -Image formatted in Interlaced order
                        pixel+1 - # bits per pixel for this image

     The specifications for the image position and size must be
confined to the dimensions defined by the Screen Descriptor.  On the
other hand it is not necessary that the image fill the entire screen
defined.

LOCAL COLOR MAP

     A Local Color Map is optional and defined here for future use. If
the 'M' bit of byte 10 of the Image Descriptor is set, then a color map
follows the Image Descriptor that applies only to the following image.
At the end of the image, the color map will revert to that defined
after the Screen Descriptor.  Note that the 'pixel' field of byte 10 of
the Image Descriptor is used only if a Local Color Map is indicated.
This defines the parameters not only for the image pixel size, but
determines the number of color map entries that follow.  The bits per
pixel value will also revert to the value specified in the Screen
Descriptor when processing of the image is complete.

RASTER DATA

     The format of the actual image is defined as the series of pixel
color index values that make up the image.  The pixels are stored left
to right sequentially for an image row.  By default each image row is
written sequentially, top to bottom.  In the case that the Interlace or
'I' bit is set in byte 10 of the Image Descriptor then the row order of
the image display follows a four-pass process in which the image is
filled in by widely spaced rows.  The first pass writes every 8th row,
starting with the top row of the image window.  The second pass writes
every 8th row starting at the fifth row from the top. The third pass
writes every 4th row starting at the third row from the top. The fourth
pass completes the image, writing every other row, starting at the
second row from the top.  A graphic description of this process
follows:

Image
Row  Pass 1  Pass 2  Pass 3  Pass 4          Result
---------------------------------------------------
0    **1a**                                  **1a**
1                            **4a**          **4a**
2                    **3a**                  **3a**
3                            **4b**          **4b**
4            **2a**                          **2a**
5                            **4c**          **4c**
6                    **3b**                  **3b**
7                            **4d**          **4d**
8    **1b**                                  **1b**
9                            **4e**          **4e**
10                   **3c**                  **3c**
11                           **4f**          **4f**
12           **2b**                          **2b**
. . .

     The image pixel values are processed as a series of color indices
which map into the existing color map.  The resulting color value from
the map is what is actually displayed.  This series of pixel indices,
the number of which is equal to image-width*image-height pixels, are
passed to the GIF image data stream one value per pixel, compressed and
packaged according to a version of the LZW compression algorithm as
defined in Appendix C.

GIF TERMINATOR

     In order to provide a synchronization for the termination of a GIF
image file, a GIF decoder will process the end of GIF mode when the
character 0x3B hex or ';' is found after an image has been processed.
By convention the decoding software will pause and wait for an action
indicating that the user is ready to continue.  This may be a carriage
return entered at the keyboard or a mouse click.  For interactive
applications this user action must be passed on to the host as a
carriage return character so that the host application can continue.
The decoding software will then typically leave graphics mode and
resume any previous process.

GIF EXTENSION BLOCKS

     To provide for orderly extension of the GIF definition, a
mechanism for defining the packaging of extensions within a GIF data
stream is necessary.  Specific GIF extensions are to be defined and
documented by CompuServe in order to provide a controlled enhancement
path.
     GIF Extension Blocks are packaged in a manner similar to that used
by the raster data though not compressed.  The basic structure is:

 7 6 5 4 3 2 1 0  Byte #
+---------------+
|0 0 1 0 0 0 0 1|  1       '!' - GIF Extension Block Introducer
+---------------+
| function code |  2       Extension function code (0 to 255)
+---------------+    ---+
|  byte count   |       |
+---------------+       |
:               :       +-- Repeated as many times as necessary
|func data bytes|       |
:               :       |
+---------------+    ---+
. . .       . . .
+---------------+
|0 0 0 0 0 0 0 0|           zero byte count (terminates block)
+---------------+

     A GIF Extension Block may immediately preceed any Image
Descriptor or occur before the GIF Terminator.  All GIF decoders must
be able to recognize the existence of GIF Extension Blocks and read
past them if unable to process the function code.  This ensures that
older decoders will be able to process extended GIF image files in the
future, though without the additional functionality.

Appendix A - Glossary

Pixel -The smallest picture element of a graphics image.  This usually
   corresponds to a single dot on a graphics screen.  Image resolution
   is typically given in units of pixels.  For example a fairly
   standard graphics screen format is one 320 pixels across and 200
   pixels high.  Each pixel can appear as one of several colors
   depending on the capabilities of the graphics hardware.
Raster -A horizontal row of pixels representing one line of an image.
   A typical method of working with images since most hardware is
   oriented to work most efficiently in this manner.
LSB - Least Significant Byte.  Refers to a convention for two byte
   numeric values in which the less significant byte of the value
   preceeds the more significant byte.  This convention is typical on
   many microcomputers.
Color Map -The list of definitions of each color used in a GIF image.
   These desired colors are converted to available colors through a
   table which is derived by assigning an incoming color index (from
   the image) to an output color index  (of the  hardware).   While the
   color map definitons are specified in a GIF image, the output  pixel
   colors will vary based on the hardware used and its ability to match
   the defined color.
Interlace - The method of displaying a GIF image in which multiple
   passes are made, outputting raster lines spaced apart to provide a
   way of visualizing the general content of an entire image  before
   all of the data has been processed.
B Protocol - A CompuServe-developed error-correcting file transfer
   protocol available in the public domain and implemented in
   CompuServe VIDTEX products.  This error checking mechanism will be
   used in transfers of GIF images for interactive applications.
LZW - A sophisticated data compression algorithm  based  on
   work done by Lempel-Ziv & Welch which has the feature of very
   efficient one-pass encoding and decoding.  This allows the image to
   be decompressed and displayed at the same  time.  The original
   article from which this technique was adapted is:
     Terry A. Welch, "A Technique for High Performance Data
     Compression", IEEE Computer, vol 17 no 6 (June 1984)
   This basic algorithm is also used in the public domain ARC file
   compression utilities.   The CompuServe adaptation of LZW for GIF is
   described in Appendix C

Appendix B -Interactive Sequences

GIF Sequence Exchanges for an Interactive Environment

     The following sequences are defined for use in  mediating
control between a GIF sender and GIF receiver over an interactive
communications line.  These sequences do not apply to applications that
involve downloading of static GIF files and are not considered part of
a GIF file.

GIF CAPABILITIES ENQUIRY

     The GCE sequence is issued from a host and requests an interactive
GIF decoder to return a response message that defines the graphics
parameters for the decoder.  This involves returning information about
available screen sizes, number of bits/color supported and the amount
of color detail supported.  The escape sequence for the GCE is defined
as:
     ESC [ > 0 g     (g is lower case, spaces inserted for clarity)
                      (0x1B 0x5B 0x3E 0x30 0x67)

GIF CAPABILITIES RESPONSE

     The GIF Capabilities Response message is returned by an
interactive GIF decoder and defines the decoder's display capabilities
for all graphics modes that are supported by the software.  Note that
this can also include graphics printers as well as a monitor screen.
The general format of this message is:

  #version;protocol{;dev, width, height, color-bits, color-res}... <CR>

'#'          - GCR identifier character (Number Sign)
version      - GIF format version number; initially '87a'
protocol='0' - No end-to-end protocol supported by decoder
               Transfer as direct 8-bit data stream.
protocol='1' - Can use an error correction protocol to transfer GIF
               data interactively from the host directly to the
               display.
dev = '0'    - Screen parameter set follows
dev = '1'    - Printer parameter set follows
width        - Maximum supported display width in pixels
height       - Maximum supported display height in pixels
color-bits   - Number of bits per pixel  supported.  The number of
               supported colors is therefore 2**color-bits.
color-res    - Number of bits per color component supported in the
               hardware color  palette.   If  color-res is '0' then no
               hardware palette table is available.

     Note that all values in the GCR are returned as ASCII decimal
numbers and the message is terminated by a Carriage Return character.

Appendix B - Interactive Sequences

     The following GCR message describes three standard EGA
configurations with no printer; the GIF data stream can be processed
within an error correcting protocol:
  #87a;1 ;0,320,200,4,0 ;0,640,200,2,2 ;0,640,350,4,2<CR>

ENTER GIF GRAPHICS MODE

     Two sequences are currently defined to invoke an interactive GIF
decoder into action.  The only difference between them is that
different output media are selected.  These sequences are:

  ESC [ > 1 g   Display GIF image on screen
                (0x1B 0x5B 0x3E 0x31 0x67)

  ESC [ > 2 g   Display image directly to an attached graphics printer.
                The  image may optionally be displayed on the screen as
                well.
                (0x1B 0x5B 0x3E 0x32 0x67)

  Note that the 'g' character terminating each sequence is in lower
case.

INTERACTIVE ENVIRONMENT

     The assumed environment for the transmission of GIF image data
from an interactive application is a full 8-bit data stream from host
to micro.  All 25 6 character codes must be transferrable.  The
establishing of an 8-bit data path for communications will normally be
taken care of by the host application programs.  It is however up to
the receiving communications programs supporting GIF to be able to
receive and pass on all 256 8-bit codes to the GIF decoder software.

Appendix C -Image Packaging & Compression

     The Raster Data stream that represents the actual output image can
be represented as:

 7 6 5 4 3 2 1 0
+---------------+
|   code size   |
+---------------+     ---+
|blok byte count|        |
+---------------+        |
:               :        +-- Repeated as many times as necessary
|  data bytes   |        |
:               :        |
+---------------+     ---+
. . .       . . .
+---------------+
|0 0 0 0 0 0 0 0| zero byte count (terminates data stream)
+---------------+

     The conversion of the image from a series of pixel values to a
transmitted or stored character stream involves several steps.  In
brief these steps are:

1.  Establish the Code Size -  Define the number of bits needed to
    represent the actual data.

2.  Compress the Data - Compress the series of image pixels to a series
    of compression codes.

3.  Build a Series of Bytes -Take the set  of compression codes and
    convert to a string of 8-bit bytes.

4.  Package the Bytes -Package sets of bytes into blocks preceeded by
    character counts and output.ESTABLISH CODE SIZE The first byte of
    the GIF Raster Data stream is a value indicating the minimum number
    of bits required to represent the set of actual pixel values.
    Normally this will be the same as the number of color bits.
    Because of some algorithmic constraints however, black & white
    images which have one color bit must be indicated as having a code
    size of 2.   This code size value also implies that the compression
    codes must start out one bit longer.

COMPRESSION

     The LZW algorithm converts a series of data values into a series
of codes which may be raw values or a code designating a series of
values.  Using text characters as an analogy, the output code consists
of a character or a code representing a string of characters.  The LZW
algorithm used in GIF matches algorithmically with the standard LZW
algorithm with the following differences:

1.  A special Clear code is defined which resets all
    compression/decompression parameters and tables to a start-up
    state.  The value of this code is 2**<code size>.  For example if
    the code size indicated was 4 (image was 4 bits/pixel) the Clear
    code value would be 16 (10000 binary).  The Clear code can appear
    at any point in the image data stream and therefore requires the
    LVW algorithm to process succeeding codes as if  a  new  data
    stream was starting.  Encoders should output a Clear code as the
    first code of each image data stream.

2.  An End of Information code is defined that explicitly indicates the
    end of the image data stream.  LZW processing terminates when this
    code is encountered.  It must be the last code output by the
    encoder for an image.  The value of this code is <Clear code>+1.

3.  The first available compression code value is <Clear code>+2.

4.  The output codes are of variable length, starting at <code size>+1
    bits per code, up to 12 bits per code.  This defines a maximum code
    value of 4095 (hex FFF).  Whenever the LZW code value would exceed
    the current  code length, the code length is increased by one.  The
    packing/unpacking of these code s must then be altered to reflect
    the new code length.

BUILD 8-BIT BYTES

     Because the LZW compression used for GIF creates a series of
variable length codes, of between 3 and 12 bits each, these codes must
be reformed into a series of 8-bit bytes that will be the characters
actually stored or transmitted.  This provides additional compression
of the image.  The codes are formed into a stream of bits as if they
were packed right to left and then picked off 8 bits at a time to be
output.   Assuming a character array of 8 bits per character and using
5 bit codes to be packed, an example layout would be similar to:

 byte n       byte 5   byte 4   byte 3   byte 2   byte 1
+-.....-----+--------+--------+--------+--------+--------+
| and so on |hhhhhggg|ggfffffe|eeeedddd|dcccccbb|bbbaaaaa|
+-.....-----+--------+--------+--------+--------+--------+

     Note that the physical packing arrangement will change as the
number of bits per compression code change but the concept remains the
same.

PACKAGE THE BYTES

     Once the bytes have been created, they are grouped into blocks for
output by preceeding each block of 0 to 255 bytes with a character
count byte.  A block with a zero byte count terminates the Raster Data
stream for a given image.  These blocks are what are actually output
for the GIF image.  This block format has the side effect of allowing a
decoding   program the ability to read past the actual image data if
necessary by reading block counts and then skipping over the data.

Appendix D -Multiple Image Processing

     Since a GIF data stream can contain multiple images, it is
necessary to describe processing and display of such a file.  Because
the image descriptor allows  for  placement of the image within the
logical screen, it is possible to define a sequence of images that may
each be a partial screen, but in total fill the entire screen.

The guidelines for handling the multiple image situation are:

1.  There is no pause between images.  Each is processed immediately as
    seen by the decoder.

2.  Each image explicitly overwrites any image already on the screen
    inside of its window.  The only screen clears are at the beginning
    and end of the GIF image process.   See discussion on the GIF
    terminator.