The Independent JPEG Group's JPEG software ========================================== README for release of 7-Oct-91 =============================== This distribution contains the first public release of the Independent JPEG Group's free JPEG software. You are welcome to redistribute this software and to use it for any purpose, subject to the conditions under LEGAL ISSUES, below. This software is still undergoing revision. Updated versions may be obtained by anonymous FTP to uunet.uu.net; look under directory /graphics/jpeg. This particular version will be archived as jpegsrc.v1.tar.Z. If you don't have access to Internet FTP, UUNET's archives are also available via UUCP; contact postmaster@uunet.uu.net for information on retrieving files that way. Please report any problems with this software to jpeg-info@uunet.uu.net. If you intend to become a serious user of this software, please contact jpeg-info@uunet to be added to our electronic mailing list. Then you'll be notified of updates and have a chance to participate in discussions, etc. This software is the work of Tom Lane, Philip Gladstone, Luis Ortiz, and other members of the independent JPEG group. DISCLAIMER ========== THIS SOFTWARE IS NOT COMPLETE NOR FULLY DEBUGGED. It is not guaranteed to be useful for anything, nor to be compatible with subsequent releases, nor to be an accurate implementation of the JPEG standard. (See LEGAL ISSUES for even more disclaimers.) WHAT'S HERE =========== This distribution contains software to implement JPEG image compression and decompression. JPEG is a standardized compression method for full-color and gray-scale images. JPEG is intended for "real-world" scenes; cartoons and other non-realistic images are not its strong suit. 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 images of real-world scenes, very good compression levels can be obtained with hardly any visible change, and amazingly high compression levels can be obtained if you can tolerate a low-quality image. For more details, see the references, or just experiment with various compression settings. The software implements JPEG baseline and extended-sequential compression processes. Provision is made for supporting all variants of these processes, although some uncommon parameter settings aren't implemented yet. For legal reasons, we are not distributing code for the arithmetic-coding process; see LEGAL ISSUES. At present we have made no provision for supporting the progressive or lossless processes defined in the standard. The present software is still largely in the prototype stage. It does not support all possible variants of the JPEG standard, and some functions have rather slow and/or crude implementations. However, it is useful already. The emphasis in designing this software has been on achieving portability and flexibility, while also making it fast enough to be useful. We have not yet undertaken serious performance measurement or tuning; we intend to do so in the future. This software can be used on several levels: * As canned software for JPEG compression and decompression. Just edit the Makefile and configuration files as needed (see SETUP), compile and go. Members of the independent JPEG group will improve the out-of-the-box functionality as time goes on. * As the basis for other JPEG programs. For example, you could incorporate the decompressor into a general image viewing package by replacing the output module with write-to-screen functions. For an implementation on specific hardware, you might want to replace some of the inner loops with assembly code. For a non-command-line-driven system, you might want a different user interface. (Members of the group will be producing Macintosh and Amiga versions with appropriate user interfaces, for example.) * As a toolkit for experimentation with JPEG and JPEG-like algorithms. Most of the individual decisions you might want to mess with are packaged up into separate modules. For example, the details of color-space conversion and subsampling techniques are each localized in one compressor and one decompressor module. You'd probably also want to extend the user interface to give you more detailed control over the JPEG compression parameters. In particular, we welcome the use of this software as the basis for commercial products; no royalty is required. SETUP ===== The installation process is not very automatic; you will need at least some familiarity with C programming and program build procedures for your system. (Volunteers to work on improving this situation are welcome. Also, we will probably start distributing pre-built binaries for popular systems at some point.) First, select a makefile and copy it to "Makefile". "makefile.unix" is appropriate for most Unix and Unix-like systems. Special makefiles are included for various PC compilers. If you don't see a makefile for your system, we recommend starting from makefile.unix. Look over the Makefile and adjust options as needed. In particular, you'll need to change the CC= and CFLAGS= definitions if you don't have gcc (makefile.unix only). If you have a function-prototype-less compiler, be sure to uncomment the .c.o rule and say "make ansi2knr". This will cause the source files to be preprocessed to change our ANSI-style function definitions to old-style definitions. (Thanks to Peter Deutsch of Aladdin Enterprises for ansi2knr.) Also look over jconfig.h and adjust #defines as necessary. If you have an ANSI-compliant C compiler (gcc for instance), no changes should be necessary except perhaps for RIGHT_SHIFT_IS_UNSIGNED and TWO_FILE_COMMANDLINE. For older compilers other mods may be needed, depending on what ANSI features are supported. If you prefer, you can usually leave jconfig.h unmodified and add -D switches to the Makefile's CFLAGS= definition. Then say "make". If you have trouble with missing system include files or inclusion of the wrong ones, you can fix it in jinclude.h. In particular, if you are using gcc on a machine with non-ANSI system include files, you are likely to find that jinclude.h tries to include the wrong files (because gcc defines __STDC__). There's no good automatic solution to this, so you'll just have to hand-edit jinclude.h. As a quick test of functionality we've included three sample files: testorig.jpg same as blkint.jpg from JPEG validation floppy. testimg.ppm output of djpeg testorig.jpg testimg.jpg output of cjpeg testimg.ppm The two .jpg files aren't identical due to different parameter choices (and wouldn't be anyway, since JPEG is lossy). However, if you can generate duplicates of testimg.ppm and testimg.jpg then you probably have a working port. "make test" will perform the necessary comparisons (by generating testout.ppm and testout.jpg and comparing these to testimg.*). NOTE: this is far from an exhaustive test of the JPEG software; some modules, such as color quantization and GIF I/O, are not exercised at all. It's just a quick test to give you some confidence that you haven't missed something major. If you need to make a smaller version of the JPEG software, some optional functions can be removed at compile time. See the xxx_SUPPORTED #defines in jconfig.h. (Not a lot is actually removed right now, but as more optional stuff gets added, this mechanism will start to make a difference.) If you want to incorporate the JPEG code as subroutines in a larger program, we recommend that you make libjpeg.a. Then use the .h files and libjpeg.a as your interface to the JPEG functions. Your surrounding program will have to provide functionality similar to what's in jcmain.c or jdmain.c, and you may want to replace jerror.c and possibly other modules depending on your needs. See the "architecture" file for more info. If it seems to you that the system structure doesn't accommodate what you want to do, please contact the authors. Special notes for Macintosh Think C users: If you have version 5.0 you should be able to just turn on __STDC__ through the compiler switch that enables that. With version 4.0 you must manually edit jconfig.h to define PROTO, HAVE_UNSIGNED_CHAR, HAVE_UNSIGNED_SHORT, and const. (It seems to be safe to just define __STDC__ to take care of the first three.) When setting up project files, use the COBJECTS and DOBJECTS lists in makefile.unix as a guide to which files need to be included, and add the ANSI and Unix C libraries in a separate segment. You may need to divide the JPEG files into more than one segment; you can do this pretty much as you please. USAGE ===== The user interface is pretty minimal at this point. We haven't bothered to generate manual-page files since the switches badly need redesign. At the moment, things work like this: There are two programs, cjpeg to compress an image file into JPEG format, and djpeg to decompress. On Unix systems, you say: cjpeg [switches] [imagefile] >jpegfile 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 PC, Macintosh, and Amiga systems, you say: cjpeg [switches] imagefile jpegfile djpeg [switches] jpegfile imagefile i.e., both 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 in jconfig.h or in the Makefile. You MUST use this style on any system that doesn't cope well with binary data fed through stdin/stdout. Currently supported image file formats include raw-format PPM, raw-format PGM (for monochrome images), and GIF. cjpeg recognizes the input image format automatically, but you have to tell djpeg which format to generate. The only JPEG file format currently supported is a raw JPEG data stream. Unless modified, the programs use the JFIF conventions for variables left unspecified by the JPEG standard. (In particular, cjpeg generates a JFIF APP0 marker.) Support for the JPEG-in-TIFF format will probably be added at some future date. The command line switches for cjpeg are: -I Generate noninterleaved JPEG file (not yet supported). -Q quality Scale quantization tables to adjust quality. Quality is 0 (worst) to 100 (best); default is 75. (See below for more info.) -a Use arithmetic coding rather than Huffman coding. (Not currently supported, see LEGAL ISSUES.) -o Perform optimization of entropy encoding parameters. Without this, default Huffman or arithmetic parameters are used. -o makes the JPEG file a tad smaller, but compression uses much more memory. Image quality is unaffected by -o. -d Enable debug printout. More -d's give more printout. Typically you'd use -Q settings of 50 or 75 or so. -Q 100 will generate a quantization table of all 1's, meaning no quantization loss; then any differences between input and output images are due to subsampling or to roundoff error in the DCT or colorspace-conversion steps. -Q values below 50 may be useful for making real small, low-quality images. Try -Q 2 (or so) for some amusing Cubist effects. (Note that -Q values below about 25 generate 2-byte quantization tables, which are not decodable by pure baseline JPEG decoders. cjpeg emits a warning message when you give such a -Q value.) The command line switches for djpeg are: -G Select GIF output format (implies -q, with default of 256 colors). -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). -g Force gray-scale output even if input is color. -q N Quantize to N colors. -D Use Floyd-Steinberg dithering in color quantization. -2 Use two-pass color quantization (not yet supported). -d Enable debug printout. More -d's give more printout. Color quantization currently uses a rather shoddy algorithm (although it's not so horrible when dithered). Because of this, the GIF output mode is not recommended in the current release, except for gray-scale output. You can get better results by applying ppmquant to the unquantized (PPM) output of djpeg, then converting to GIF with ppmtogif. We expect to provide a considerably better quantization algorithm in a future release. Note that djpeg *can* read noninterleaved JPEG files even though cjpeg can't yet generate them. For most applications this is a nonissue, since hardly anybody seems to be using noninterleaved format. On a non-virtual-memory machine, you may run out of memory if you use -I or -o in cjpeg, or -q ... -2 in djpeg, or try to read an interlaced GIF file. This will be addressed eventually by replacing jvirtmem.c with something that uses temporary files for large images (see TO DO). REFERENCES ========== The best and most readily available introduction to the JPEG compression algorithm is Wallace's article in the April '91 CACM: Wallace, Gregory K. "The JPEG Still Picture Compression Standard", Communications of the ACM, April 1991 (vol. 34 no. 4), pp. 30-44. (Adjacent articles in that issue discuss MPEG motion picture compression, applications of JPEG, and related topics.) We highly recommend reading that article before looking at any of the JPEG software. For more detail about the JPEG standard you pretty much have to go to the draft standard, which is not nearly as intelligible as Wallace's article. The current version is ISO/IEC Committee Draft CD 10918-1 dated 1991-03-15. The standard is not presently available electronically; you must order a paper copy through ISO. The JPEG standard does not specify all details of an interchangeable file format. For the omitted details we follow the "JFIF" conventions, revision 1.01. A copy of the JFIF spec is available from: Literature Department C-Cube Microsystems, Inc. 399A West Trimble Road San Jose, CA 95131 (408) 944-6300 Requests can also be e-mailed to info@c3.pla.ca.us (this address good after 10/10/91). The same source can supply copies of the draft JPEG-in-TIFF specs. If you want to understand this implementation, start by reading the "architecture" documentation file. Please read "codingrules" if you want to contribute any code. SUPPORTING SOFTWARE =================== You will probably want Jef Poskanzer's PBMPLUS image software; this provides many useful operations on PPM-format image files. In particular, it can convert PPM images to and from a wide range of other formats. You can FTP this free software from export.lcs.mit.edu (contrib/pbmplus*.tar.Z) or ftp.ee.lbl.gov (pbmplus*.tar.Z). If you are using X Windows you might want to use the xv or xloadimage viewers to save yourself the trouble of converting PPM to some other format. Both of these can be found in the contrib directory at export.lcs.mit.edu. LEGAL ISSUES ============ The authors make NO WARRANTY or representation, either express or implied, with respect to this software, its quality, accuracy, merchantability, or fitness for a particular purpose. This software is provided "AS IS", and you, its user, assume the entire risk as to its quality and accuracy. This software is copyright (C) 1991, Thomas G. Lane. All Rights Reserved except as specified below. Permission is hereby granted to use, copy, modify, and distribute this software (or portions thereof) for any purpose, without fee, subject to these conditions: (1) If any part of the source code for this software is distributed, then this README file must be included, with this copyright and no-warranty notice unaltered; and any additions, deletions, or changes to the original files must be clearly indicated in accompanying documentation. (2) If only executable code is distributed, then the accompanying documentation must state that "this software is based in part on the work of the Independent JPEG Group". (3) Permission for use of this software is granted only if the user accepts full responsibility for any undesirable consequences; the authors accept NO LIABILITY for damages of any kind. Permission is NOT granted for the use of any author's name or author's company name in advertising or publicity relating to this software or products derived from it. This software may be referred to only as "the Independent JPEG Group's software". We specifically permit and encourage the use of this software as the basis of commercial products, provided that all warranty or liability claims are assumed by the product vendor. ansi2knr.c is included in this distribution by permission of L. Peter Deutsch, sole proprietor of its copyright holder, Aladdin Enterprises of Menlo Park, CA. ansi2knr.c is NOT covered by the above copyright and conditions, but instead by the usual distribution terms of the Free Software Foundation; principally, that you must include source code if you redistribute it. (See the file ansi2knr.c for full details.) However, since ansi2knr.c is not needed as part of any product generated from the JPEG code, this does not limit you more than the foregoing paragraphs do. It appears that the arithmetic coding option of the JPEG spec is covered by patents held by IBM, and possibly also patents of AT&T and Mitsubishi. Hence arithmetic coding cannot legally be used without obtaining one or more licenses. For this reason, support for arithmetic coding has been removed from the free JPEG software. (Since arithmetic coding provides only a marginal gain over the unpatented Huffman mode, it is unlikely that very many people will choose to use it. If you do obtain such a license, contact jpeg-info@uunet.uu.net for a copy of our arithmetic coding modules.) So far as we are aware, there are no patent restrictions on the remaining code. TO DO ===== Many of the modules need fleshing out to provide more complete implementations, or to provide faster paths for common cases. The greatest needs are for (a) decent color quantization, and (b) a memory manager implementation that can work in limited memory by swapping "big" images to temporary files. I (Tom Lane) am going to work on color quantization next. Volunteers to write a PC memory manager, or to work on any other modules, are welcome. We'd appreciate it if people would compile and check out the code on as wide a variety of systems as possible, and report any portability problems encountered (with solutions, if possible). Checks of file compatibility with other JPEG implementations would also be of interest. Finally, we would appreciate code profiles showing where the most time is spent, especially on unusual systems. Please send bug reports, offers of help, etc. to jpeg-info@uunet.uu.net.