The Very Best of Atari Inside

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USAGE instructions for the Independent JPEG Group's JPEG software ================================================================= INTRODUCTION This distribution contains software to implement JPEG image compression and decompression. JPEG (pronounced "jay-peg") is a standardized compression method for full-color and gray-scale images. JPEG is designed to handle "real-world" scenes, for example scanned photographs. Cartoons, line drawings, and other non-realistic images are not JPEG's strong suit; on this sort of material you may get poor image quality and/or little compression. JPEG is lossy, meaning that the output image is not necessarily identical to the input image. Hence you should not use JPEG if you have to have identical output bits. However, on typical real-world images, very good compression levels can be obtained with no visible change, and amazingly high compression is possible if you can tolerate a low-quality image. You can trade off image quality against file size by adjusting the compressor's "quality" setting. This file describes usage of the standard programs "cjpeg" and "djpeg" that can be built directly from the distributed C code. See the README file for hints on incorporating the JPEG software into other programs. If you are on a Unix machine you may prefer to read the Unix-style manual pages in files cjpeg.1 and djpeg.1. NOTE: at some point we will probably redesign the user interface, so the command line switches described here will change. GENERAL USAGE We provide two programs, cjpeg to compress an image file into JPEG format, and djpeg to decompress a JPEG file back into a conventional image format. On Unix-like systems, you say: cjpeg [switches] [imagefile] >jpegfile or djpeg [switches] [jpegfile] >imagefile The programs read the specified input file, or standard input if none is named. They always write to standard output (with trace/error messages to standard error). These conventions are handy for piping images between programs. On most non-Unix systems, you say: cjpeg [switches] imagefile jpegfile or djpeg [switches] jpegfile imagefile i.e., both the input and output files are named on the command line. This style is a little more foolproof, and it loses no functionality if you don't have pipes. (You can get this style on Unix too, if you prefer, by defining TWO_FILE_COMMANDLINE when you compile the programs; see SETUP.) The currently supported image file formats are: PPM (PBMPLUS color format), PGM (PBMPLUS gray-scale format), GIF, Targa, and RLE (Utah Raster Toolkit format). (RLE is supported only if the URT library is available.) cjpeg recognizes the input image format automatically, with the exception of some Targa-format files. You have to tell djpeg which format to generate. The only JPEG file format currently supported is the JFIF format. Support for the TIFF/JPEG format will probably be added at some future date. CJPEG DETAILS The command line switches for cjpeg are: -Q quality Scale quantization tables to adjust image quality. Quality is 0 (worst) to 100 (best); default is 75. (See below for more info.) -o Perform optimization of entropy encoding parameters. Without this, default encoding parameters are used. -o usually makes the JPEG file a little smaller, but cjpeg runs somewhat slower and needs much more memory. Image quality and speed of decompression are unaffected by -o. -T Input file is Targa format. Targa files that contain an "identification" field will not be automatically recognized by cjpeg; for such files you must specify -T to force cjpeg to treat the input as Targa format. -I Generate noninterleaved JPEG file (not yet supported). -a Use arithmetic coding rather than Huffman coding. (Not currently supported for legal reasons.) -d Enable debug printout. More -d's give more printout. Also, version information is printed at startup. -m memory Set limit for amount of memory to use in processing large images. Value is in thousands of bytes, or millions of bytes if "M" is attached to the number. For example, -m 4m selects 4000000 bytes. If more space is needed, temporary files will be used. The -Q switch lets you trade off compressed file size against quality of the reconstructed image: the higher the -Q setting, the larger the JPEG file, and the closer the output image will be to the original input. Normally you want to use the lowest -Q setting (smallest file) that decompresses into something visually indistinguishable from the original image. For this purpose the -Q setting should be between 50 and 95; the default of 75 is often about right. If you see defects at -Q 75, then go up 5 or 10 counts at a time until you are happy with the output image. (The optimal setting will vary from one image to another.) -Q 100 will generate a quantization table of all 1's, eliminating loss in the quantization step (but there is still information loss in subsampling, as well as roundoff error). This setting is mainly of interest for experimental purposes. -Q values above about 95 are NOT recommended for normal use; the compressed file size goes up dramatically for hardly any gain in output image quality. In the other direction, -Q values below 50 will produce very small files of low image quality. Settings around 5 to 10 might be useful in preparing an index of a large image library, for example. Try -Q 2 (or so) for some amusing Cubist effects. (Note: -Q values below about 25 generate 2-byte quantization tables, which are considered optional in the JPEG standard. cjpeg emits a warning message when you give such a -Q value, because some commercial JPEG programs may be unable to decode the resulting file.) DJPEG DETAILS The command line switches for djpeg are: -G Select GIF output format (implies -q, with default of 256 colors). -P Select PPM or PGM output format (this is the default). PGM is emitted if the JPEG file is gray-scale or if -g is specified. -R Select RLE output format. Requires URT library. -T Select Targa output format. Gray-scale format is emitted if the JPEG file is gray-scale or if -g is specified; otherwise, colormapped format is emitted if -q is specified; otherwise, 24-bit full-color format is emitted. -g Force gray-scale output even if input is color. -q N Quantize to N colors. This reduces the number of colors in the output image so that it can be displayed on a colormapped display or stored in a colormapped file format. For example, if you have an 8-bit display, you'd need to quantize to 256 or fewer colors. -D Do not use dithering in color quantization. By default, Floyd-Steinberg dithering is applied when quantizing colors, but on some images dithering may result in objectionable "graininess". If that happens, you can turn off dithering with -D. -D is ignored unless you also say -q or -G. -1 Use one-pass instead of two-pass color quantization. The one-pass method is faster and needs less memory, but it produces a lower-quality image. -1 is ignored unless you also say -q or -G. Also, the one-pass method is always used for gray-scale output (the two-pass method is no improvement then). -b Perform cross-block smoothing. This is quite memory-intensive and only seems to improve the image at very low quality settings (-Q 10 to 20 or so). At normal -Q settings it may make the image worse. -d Enable debug printout. More -d's give more printout. Also, version information is printed at startup. -m memory Set limit for amount of memory to use in processing large images. Value is in thousands of bytes, or millions of bytes if "M" is attached to the number. For example, -m 4m selects 4000000 bytes. If more space is needed, temporary files will be used. HINTS Avoid running an image through a series of JPEG compression/decompression cycles. Image quality loss will accumulate; after ten or so cycles the image may be noticeably worse than it was after one cycle. It's best to use a lossless format while manipulating an image, then convert to JPEG format when you are ready to file the image away. The -o option to cjpeg is worth using when you are making a "final" version for posting or archiving. It's also a win when you are using low -Q settings to make very small JPEG files; the percentage improvement is often a lot more than it is on larger files. The default memory usage limit (-m) is set when the software is compiled. If you get an "insufficient memory" error, try specifying a smaller -m value, even -m 0 to use the absolute minimum space. You may want to recompile with a smaller default value if this happens often. djpeg with two-pass color quantization requires a good deal of space; on MS-DOS machines it may run out of memory even with -m 0. In that case you can still decompress, with some loss of image quality, by specifying -1 for one-pass quantization. If more space is needed than will fit in the available main memory (as determined by -m), temporary files will be used. (MS-DOS versions will try to get extended or expanded memory first.) The temporary files are often rather large: in typical cases they occupy three bytes per pixel, for example 3*800*600 = 1.44Mb for an 800x600 image. If you don't have enough free disk space, leave out -o (for cjpeg) or specify -1 (for djpeg). On MS-DOS, the temporary files are created in the directory named by the TMP or TEMP environment variable, or in the current directory if neither of those exist. Amiga implementations put the temp files in the directory named by JPEGTMP:, so be sure to assign JPEGTMP: to a disk partition with adequate free space.