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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. NOTE: the switch syntax has been redesigned since the v3 release of cjpeg/djpeg. Switch names are now words instead of single letters. Also note that this DOS version uses a different command-line syntax than the Unix version. 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. The basic command line is: cjpeg [switches] list of image files or djpeg [switches] list of jpeg files Each file named is compressed or decompressed. The input file(s) are not modified; the output data is written to files which have the same names except for extension. cjpeg always uses ".jpg" for the output file name's extension; djpeg uses one of ".gif", ".ppm", or ".tga", depending on what output format is selected by the switches. For example, to convert xxx.gif to xxx.jpg and yyy.ppm to yyy.jpg, say: cjpeg xxx.gif yyy.ppm On most systems you can use standard wildcards to specify the list of input files; for example, on DOS "djpeg *.jpg" decompresses all the JPEG files in the current directory. If an intended output file already exists, you'll be asked whether or not to overwrite it. If you say no, the program skips that input file and goes on to the next one. You can intermix switches and file names; for example djpeg -gif file1.jpg -targa file2.jpg decompresses file1.jpg into GIF format (file1.gif) and file2.jpg into Targa format (file2.tga). Only switches to the left of a given file name affect processing of that file; when there are conflicting switches, the rightmost one takes precedence. 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, which it isn't on most non-Unix systems.) cjpeg recognizes the input image format automatically, with the exception of some Targa-format files. You have to tell djpeg which output format to generate. The only JPEG file format currently supported is the JFIF format. Support for the TIFF 6.0 JPEG format will probably be added at some future date. All switch names may be abbreviated; for example, -grayscale may be written -gray or -gr. Most of the "basic" switches can be abbreviated to as little as one letter. Upper and lower case are equivalent (-GIF is the same as -gif). British spellings are also accepted (e.g., -greyscale), though for brevity these are not mentioned below. CJPEG DETAILS The basic command line switches for cjpeg are: -quality N Scale quantization tables to adjust image quality. Quality is 0 (worst) to 100 (best); default is 75. (See below for more info.) -grayscale Create monochrome JPEG file from color input. Be sure to use this switch when compressing a grayscale GIF file, because cjpeg isn't bright enough to notice whether a GIF file uses only shades of gray. By saying -grayscale, you'll get a smaller JPEG file that takes less time to process. -optimize Perform optimization of entropy encoding parameters. Without this, default encoding parameters are used. -optimize 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 -optimize. -targa 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 -targa to make cjpeg treat the input as Targa format. The -quality switch lets you trade off compressed file size against quality of the reconstructed image: the higher the quality 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 quality setting (smallest file) that decompresses into something visually indistinguishable from the original image. For this purpose the quality setting should be between 50 and 95; the default of 75 is often about right. If you see defects at -quality 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.) -quality 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. Quality 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, quality 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 -quality 2 (or so) for some amusing Cubist effects. (Note: quality 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 quality value, because some commercial JPEG programs may be unable to decode the resulting file.) Switches for advanced users: -maxmemory N 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, -max 4m selects 4000000 bytes. If more space is needed, temporary files will be used. -restart N Emit a JPEG restart marker every N MCU rows, or every N MCU blocks if "B" is attached to the number. -restart 0 (the default) means no restart markers. -smooth N Smooth the input image to eliminate dithering noise. N, ranging from 1 to 100, indicates the strength of smoothing. 0 (the default) means no smoothing. -verbose Enable debug printout. More -v's give more printout. or -debug Also, version information is printed at startup. The -restart option inserts extra markers that allow a JPEG decoder to resynchronize after a transmission error. Without restart markers, any damage to a compressed file will usually ruin the image from the point of the error to the end of the image; with restart markers, the damage is usually confined to the portion of the image up to the next restart marker. Of course, the restart markers occupy extra space. We recommend -restart 1 for images that will be transmitted across unreliable networks such as Usenet. The -smooth option filters the input to eliminate fine-scale noise. This is often useful when converting GIF files to JPEG: a moderate smoothing factor of 10 to 50 gets rid of dithering patterns in the input file, resulting in a smaller JPEG file and a better-looking image. Too large a smoothing factor will visibly blur the image, however. Switches for wizards: -arithmetic Use arithmetic coding rather than Huffman coding. (Not currently supported for legal reasons.) -nointerleave Generate noninterleaved JPEG file (not yet supported). -qtables file Use the quantization tables given in the specified file. The file should contain one to four tables (64 values each) as plain text. Comments preceded by '#' may be included in the file. The tables are implicitly numbered 0,1,etc. If -quality N is also specified, the values in the file are scaled according to cjpeg's quality scaling curve. -sample HxV[,...] Set JPEG sampling factors. If you specify fewer H/V pairs than there are components, the remaining components are set to 1x1 sampling. The default setting is equivalent to "-sample 2x2". The "wizard" switches are intended for experimentation with JPEG. If you don't know what you are doing, DON'T USE THEM. You can easily produce files with worse image quality and/or poorer compression than you'll get from the default settings. Furthermore, these switches should not be used when making files intended for general use, because not all JPEG implementations will support unusual JPEG parameter settings. DJPEG DETAILS The basic command line switches for djpeg are: -colors N Reduce image to at most N colors. This reduces the or -quantize N number of colors used 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 reduce to 256 or fewer colors. (-colors is the recommended name, -quantize is provided only for backwards compatibility.) -gif Select GIF output format (this is the default format). Since GIF does not support more than 256 colors, -colors 256 is assumed (unless you specify a smaller number of colors). -pnm Select PBMPLUS (PPM/PGM) output format. PGM is emitted if the JPEG file is gray-scale or if -grayscale is specified; otherwise PPM is emitted. -targa Select Targa output format. Gray-scale format is emitted if the JPEG file is gray-scale or if -grayscale is specified; otherwise, colormapped format is emitted if -colors is specified; otherwise, 24-bit full-color format is emitted. Switches for advanced users: -blocksmooth Perform cross-block smoothing. This is quite memory-intensive and only seems to improve the image at very low quality settings (-quality 10 to 20 or so). At normal quality settings it may make things worse. -grayscale Force gray-scale output even if JPEG file is color. Useful for viewing on monochrome displays. -maxmemory N 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, -max 4m selects 4000000 bytes. If more space is needed, temporary files will be used. -nodither 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 -nodither. -nodither is ignored unless you also say -colors N. -onepass 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. -onepass is ignored unless you also say -colors N. Also, the one-pass method is always used for gray-scale output (the two-pass method is no improvement then). -verbose Enable debug printout. More -v's give more printout. or -debug Also, version information is printed at startup. HINTS Color GIF files are not the ideal input for JPEG; JPEG is really intended for compressing full-color (24-bit) images. In particular, don't try to convert cartoons, line drawings, and other images that have only a few distinct colors. GIF works great on these, JPEG does not. If you want to convert a GIF to JPEG, you should experiment with cjpeg's -quality and -smooth options to get a satisfactory conversion. -smooth 10 or so is often helpful. 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 -optimize 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 quality settings to make very small JPEG files; the percentage improvement is often a lot more than it is on larger files. When making images to be posted on Usenet, we recommend using cjpeg's option -restart 1. This option limits the damage done to a compressed image by netnews transmission errors. djpeg with two-pass color quantization may run out of memory on very wide images. In that case you can still decompress, with some loss of image quality, by specifying -onepass for one-pass quantization. If more space is needed than will fit in the available main memory, 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 -optimize (for cjpeg) or specify -onepass (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.