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The Independent JPEG Group’s JPEG software README for release 6b of
This distribution contains the sixth 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.
Serious users of this software (particularly those incorporating it into larger programs) should contact IJG at
This software is the work of Tom Lane, Philip Gladstone, Jim Boucher, Lee Crocker, Julian Minguillon, Luis Ortiz, George Phillips, Davide Rossi, Guido Vollbeding, Ge’ Weijers, and other members of the Independent JPEG Group.
IJG is not affiliated with the official ISO JPEG standards committee.
DOCUMENTATION ROADMAP
This file contains the following sections:
OVERVIEW | General description of JPEG and the IJG software. |
LEGAL ISSUES | Copyright, lack of warranty, terms of distribution. |
REFERENCES | Where to learn more about JPEG. |
ARCHIVE LOCATIONS | Where to find newer versions of this software. |
RELATED SOFTWARE | Other stuff you should get. |
FILE FORMAT WARS | Software *not* to get. |
TO DO | Plans for future IJG releases. |
Other documentation files in the distribution are: | |
User documentation: |
|
install.doc | How to configure and install the IJG software. |
usage.doc | Usage instructions for cjpeg, djpeg, jpegtran, rdjpgcom, |
| and wrjpgcom. |
*.1 | |
wizard.doc | Advanced usage instructions for JPEG wizards only. |
change.log | |
Programmer and internal documentation: | |
libjpeg.doc | How to use the JPEG library in your own programs. |
example.c | Sample code for calling the JPEG library. |
structure.doc | Overview of the JPEG library’s internal structure. |
filelist.doc | Road map of IJG files. |
coderules.doc | Coding style rules |
Please read at least the files install.doc and usage.doc. Useful information can also be found in the JPEG FAQ (Frequently Asked Questions) article. See ARCHIVE LOCATIONS below to find out where to obtain the FAQ article.
If you want to understand how the JPEG code works, we suggest reading one or more of the REFERENCES, then looking at the documentation files (in roughly the order listed) before diving into the code.
OVERVIEW
This package contains C software to implement JPEG image compression and decompression. JPEG (pronounced
This software implements JPEG baseline,
We provide a set of library routines for reading and writing JPEG image files, plus two sample applications “cjpeg” and “djpeg”, which use the library to perform conversion between JPEG and some other popular image file formats. The library is intended to be reused in other applications.
In order to support file conversion and viewing software, we have included considerable functionality beyond the bare JPEG coding/decoding capability; for example, the color quantization modules are not strictly part of JPEG decoding, but they are essential for output to colormapped file formats or colormapped displays. These extra functions can be compiled out of the library if not required for a particular application. We have also included jpegtran, a utility for lossless transcoding between different JPEG processes, and “rdjpgcom” and “wrjpgcom”, two simple applications for inserting and extracting textual comments in JFIF files. The emphasis in designing this software has been on achieving portability and flexibility, while also making it fast enough to be useful. In particular, the software is not intended to be read as a tutorial on JPEG. (See the REFERENCES section for introductory material.) Rather, it is intended to be reliable, portable, industrial- strength code. We do not claim to have achieved that goal in every aspect of the software, but we strive for it.
We welcome the use of this software as a component of commercial products. No royalty is required, but we do ask for an acknowledgement in product documentation, as described under LEGAL ISSUES.
LEGAL ISSUES In plain English:
1.We don’t promise that this software works. (But if you find any bugs, please let us know!)
2.You can use this software for whatever you want. You don’t have to pay us.
3.You may not pretend that you wrote this software. If you use it in a program, you must acknowledge somewhere in your documentation that you’ve used the IJG code.
In legalese:
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 ©
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
(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.
These conditions apply to any software derived from or based on the IJG code, not just to the unmodified library. If you use our work, you ought to acknowledge us. Permission is NOT granted for the use of any IJG author's name or 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 program generated from the IJG code, this does not limit you more than the foregoing paragraphs do. The Unix configuration script “configure” was produced with GNU Autoconf. It is copyright by the Free Software Foundation but is freely distributable. The same holds for its supporting scripts (config.guess, config.sub, ltconfig, ltmain.sh). Another support script,
It appears that the arithmetic coding option of the JPEG spec is covered by patents owned by IBM, 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 implementations will support it.) So far as we are aware, there are no patent restrictions on the remaining code.
The IJG distribution formerly included code to read and write GIF files. To avoid entanglement with the Unisys LZW patent, GIF reading support has been removed altogether, and the GIF writer has been simplified to produce uncompressed GIFs. This technique does not use the LZW algorithm; the resulting GIF files are larger than usual, but are readable by all standard GIF decoders.
We are required to state that
“The Graphics Interchange Format(c) is the Copyright property of CompuServe Incorporated. GIF(sm) is a Service Mark property of CompuServe Incorporated.”
REFERENCES
We highly recommend reading one or more of these references before trying to understand the innards of the JPEG software.
The best short technical introduction to the JPEG compression algorithm is Wallace, Gregory K. “The JPEG Still Picture Compression Standard”, Communications of the ACM, April 1991 (vol. 34 no. 4), pp.
applications of JPEG, and related topics.) If you don’t have the CACM issue handy, a PostScript file containing a revised version of Wallace’s article is available at ftp:/ /ftp.uu.net/graphics/jpeg/wallace.ps.gz. The file (actually a preprint for an article that appeared in IEEE Trans. Consumer Electronics) omits the sample images that appeared in CACM, but it includes corrections and some added material. Note: the
