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Text File | 1994-12-12 | 21KB | 453 lines

USAGE instructions for the Independent JPEG Group's JPEG software ================================================================= This file describes usage of the JPEG conversion programs cjpeg and djpeg, as well as the utility programs rdjpgcom and wrjpgcom. (See the other documentation files if you wish to use the JPEG library within your own programs.) If you are on a Unix machine you may prefer to read the Unix-style manual pages in files cjpeg.1, djpeg.1, rdjpgcom.1, wrjpgcom.1. INTRODUCTION These programs 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 that 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. 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 install.doc.) You can also say: cjpeg [switches] -outfile jpegfile imagefile or djpeg [switches] -outfile imagefile jpegfile This syntax works on all systems, so it is useful for scripts. The currently supported image file formats are: PPM (PBMPLUS color format), PGM (PBMPLUS gray-scale format), BMP, 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. JPEG files are in the defacto standard JFIF file format. There are other, less widely used JPEG-based file formats, but we don't support them. 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. For most Targa files, you won't need this switch. 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. Use -baseline if you need to ensure compatibility at low quality values.) Switches for advanced users: -dct int Use integer DCT method (default). -dct fast Use fast integer DCT (less accurate). -dct float Use floating-point DCT method. The float method is very slightly more accurate than the int method, but is much slower unless your machine has very fast floating-point hardware. Also note that results of the floating-point method may vary slightly across machines, while the integer methods should give the same results everywhere. The fast integer method is much less accurate than the other two. -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. -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. -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.) -baseline Force a baseline JPEG file to be generated. This clamps quantization values to 8 bits even at low quality settings. -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. -qslots N[,...] Select which quantization table to use for each color component. By default, table 0 is used for luminance and table 1 for chrominance components. -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.) -fast Select recommended processing options for fast, low quality output. (The default options are chosen for highest quality output.) Currently, this is equivalent to "-dct fast -nosmooth -onepass -dither ordered". -grayscale Force gray-scale output even if JPEG file is color. Useful for viewing on monochrome displays; also, djpeg runs noticeably faster in this mode. -scale M/N Scale the output image by a factor M/N. Currently the scale factor must be 1/1, 1/2, 1/4, or 1/8. Scaling is handy if the image is larger than your screen; also, djpeg runs much faster when scaling down the output. -bmp Select BMP output format (Windows flavor). 8-bit colormapped format is emitted if -colors or -grayscale is specified, or if the JPEG file is gray-scale; otherwise, 24-bit full-color format is emitted. -gif Select GIF output format. Since GIF does not support more than 256 colors, -colors 256 is assumed (unless you specify a smaller number of colors). If you specify -fast, the default number of colors is 216. -os2 Select BMP output format (OS/2 1.x flavor). 8-bit colormapped format is emitted if -colors or -grayscale is specified, or if the JPEG file is gray-scale; otherwise, 24-bit full-color format is emitted. -pnm Select PBMPLUS (PPM/PGM) output format (this is the default format). PGM is emitted if the JPEG file is gray-scale or if -grayscale is specified; otherwise PPM is emitted. -rle Select RLE output format. (Requires URT library.) -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: -dct int Use integer DCT method (default). -dct fast Use fast integer DCT (less accurate). -dct float Use floating-point DCT method. The float method is very slightly more accurate than the int method, but is much slower unless your machine has very fast floating-point hardware. Also note that results of the floating-point method may vary slightly across machines, while the integer methods should give the same results everywhere. The fast integer method is much less accurate than the other two. -dither fs Use Floyd-Steinberg dithering in color quantization. -dither ordered Use ordered dithering in color quantization. -dither none Do not use dithering in color quantization. By default, Floyd-Steinberg dithering is applied when quantizing colors; this is slow but usually produces the best results. Ordered dither is a compromise between speed and quality; no dithering is fast but usually looks awful. Note that these switches have no effect unless color quantization is being done. Ordered dither is only available in -onepass mode. -map FILE Quantize to the colors used in the specified image file. This is useful for producing multiple files with identical color maps, or for forcing a predefined set of colors to be used. The FILE must be a GIF or PPM file. This option overrides -colors and -onepass. -nosmooth Use a faster, lower-quality upsampling routine. -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). -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. -verbose Enable debug printout. More -v's give more printout. or -debug Also, version information is printed at startup. HINTS FOR CJPEG 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. HINTS FOR DJPEG To get a quick preview of an image, use the -grayscale and/or -scale switches. "-grayscale -scale 1/8" is the fastest case. Several options are available that trade off image quality to gain speed. "-fast" turns on the recommended settings. "-dct fast" and/or "-nosmooth" gain speed at a small sacrifice in quality. When producing a color-quantized image, "-onepass -dither ordered" is fast but much lower quality than the default behavior. "-dither none" may give acceptable results in two-pass mode, but is seldom tolerable in one-pass mode. If you are fortunate enough to have very fast floating point hardware, "-dct float" may be even faster than "-dct fast". But on most machines "-dct float" is slower than "-dct int"; in this case it is not worth using, because its theoretical accuracy advantage is too small to be significant in practice. Two-pass color quantization requires a good deal of memory; on MS-DOS machines it may run out of memory even with -maxmemory 0. In that case you can still decompress, with some loss of image quality, by specifying -onepass for one-pass quantization. HINTS FOR BOTH PROGRAMS If more space is needed than will fit in the available main memory (as determined by -maxmemory), 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. 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. The default memory usage limit (-maxmemory) is set when the software is compiled. If you get an "insufficient memory" error, try specifying a smaller -maxmemory value, even -maxmemory 0 to use the absolute minimum space. You may want to recompile with a smaller default value if this happens often. On machines that have "environment" variables, you can define the environment variable JPEGMEM to set the default memory limit. The value is specified as described for the -maxmemory switch. JPEGMEM overrides the default value specified when the program was compiled, and itself is overridden by an explicit -maxmemory switch. On MS-DOS machines, -maxmemory is the amount of main (conventional) memory to use. (Extended or expanded memory is also used if available.) Most DOS-specific versions of this software do their own memory space estimation and do not need you to specify -maxmemory. THE COMMENT UTILITIES The JPEG standard allows "comment" (COM) blocks to occur within a JPEG file. Although the standard doesn't actually define what COM blocks are for, they are widely used to hold user-supplied text strings. This lets you add annotations, titles, index terms, etc to your JPEG files, and later retrieve them as text. COM blocks do not interfere with the image stored in the JPEG file. The maximum size of a COM block is 64K, but you can have as many of them as you like in one JPEG file. We provide two utility programs to display COM block contents and add COM blocks to a JPEG file. rdjpgcom searches a JPEG file and prints the contents of any COM blocks on standard output. The command line syntax is rdjpgcom [-verbose] [inputfilename] The switch "-verbose" (or just "-v") causes rdjpgcom to also display the JPEG image dimensions. If you omit the input file name from the command line, the JPEG file is read from standard input. (This may not work on some operating systems, if binary data can't be read from stdin.) wrjpgcom adds a COM block, containing text you provide, to a JPEG file. Ordinarily, the COM block is added after any existing COM blocks, but you can delete the old COM blocks if you wish. wrjpgcom produces a new JPEG file; it does not modify the input file. DO NOT try to overwrite the input file by directing wrjpgcom's output back into it; on most systems this will just destroy your file. The command line syntax for wrjpgcom is similar to cjpeg's. On Unix-like systems, it is wrjpgcom [switches] [inputfilename] The output file is written to standard output. The input file comes from the named file, or from standard input if no input file is named. On most non-Unix systems, the syntax is wrjpgcom [switches] inputfilename outputfilename where both input and output file names must be given explicitly. wrjpgcom understands three switches: -replace Delete any existing COM blocks from the file. -comment "Comment text" Supply new COM text on command line. -cfile name Read text for new COM block from named file. (Switch names can be abbreviated.) If you have only one line of comment text to add, you can provide it on the command line with -comment. The comment text must be surrounded with quotes so that it is treated as a single argument. Longer comments can be read from a text file. If you give neither -comment nor -cfile, then wrjpgcom will read the comment text from standard input. (In this case an input image file name MUST be supplied, so that the source JPEG file comes from somewhere else.) You can enter multiple lines, up to 64KB worth. Type an end-of-file indicator (usually control-D or control-Z) to terminate the comment text entry. wrjpgcom will not add a COM block if the provided comment string is empty. Therefore -replace -comment "" can be used to delete all COM blocks from a file. These utility programs do not depend on the IJG JPEG library. In particular, the source code for rdjpgcom is intended as an illustration of the minimum amount of code required to parse a JPEG file header correctly.