The Independent JPEG Group's JPEG software+
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==========================================+
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+
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README for release 6b of 27-Mar-1998+
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====================================+
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+
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This distribution contains the sixth public release of the Independent JPEG+
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Group's free JPEG software.  You are welcome to redistribute this software and+
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to use it for any purpose, subject to the conditions under LEGAL ISSUES, below.+
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+
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Serious users of this software (particularly those incorporating it into+
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larger programs) should contact IJG at jpeg-info@uunet.uu.net to be added to+
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our electronic mailing list.  Mailing list members are notified of updates+
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and have a chance to participate in technical discussions, etc.+
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+
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This software is the work of Tom Lane, Philip Gladstone, Jim Boucher,+
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Lee Crocker, Julian Minguillon, Luis Ortiz, George Phillips, Davide Rossi,+
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Guido Vollbeding, Ge' Weijers, and other members of the Independent JPEG+
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Group.+
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+
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IJG is not affiliated with the official ISO JPEG standards committee.+
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+
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+
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DOCUMENTATION ROADMAP+
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=====================+
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+
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This file contains the following sections:+
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+
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OVERVIEW            General description of JPEG and the IJG software.+
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LEGAL ISSUES        Copyright, lack of warranty, terms of distribution.+
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REFERENCES          Where to learn more about JPEG.+
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ARCHIVE LOCATIONS   Where to find newer versions of this software.+
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RELATED SOFTWARE    Other stuff you should get.+
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FILE FORMAT WARS    Software *not* to get.+
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TO DO               Plans for future IJG releases.+
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+
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Other documentation files in the distribution are:+
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+
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User documentation:+
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  install.doc       How to configure and install the IJG software.+
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  usage.doc         Usage instructions for cjpeg, djpeg, jpegtran,+
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                    rdjpgcom, and wrjpgcom.+
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  *.1               Unix-style man pages for programs (same info as usage.doc).+
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  wizard.doc        Advanced usage instructions for JPEG wizards only.+
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  change.log        Version-to-version change highlights.+
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Programmer and internal documentation:+
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  libjpeg.doc       How to use the JPEG library in your own programs.+
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  example.c         Sample code for calling the JPEG library.+
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  structure.doc     Overview of the JPEG library's internal structure.+
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  filelist.doc      Road map of IJG files.+
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  coderules.doc     Coding style rules --- please read if you contribute code.+
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+
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Please read at least the files install.doc and usage.doc.  Useful information+
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can also be found in the JPEG FAQ (Frequently Asked Questions) article.  See+
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ARCHIVE LOCATIONS below to find out where to obtain the FAQ article.+
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+
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If you want to understand how the JPEG code works, we suggest reading one or+
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more of the REFERENCES, then looking at the documentation files (in roughly+
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the order listed) before diving into the code.+
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+
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+
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OVERVIEW+
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========+
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+
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This package contains C software to implement JPEG image compression and+
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decompression.  JPEG (pronounced "jay-peg") is a standardized compression+
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method for full-color and gray-scale images.  JPEG is intended for compressing+
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"real-world" scenes; line drawings, cartoons and other non-realistic images+
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are not its strong suit.  JPEG is lossy, meaning that the output image is not+
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exactly identical to the input image.  Hence you must not use JPEG if you+
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have to have identical output bits.  However, on typical photographic images,+
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very good compression levels can be obtained with no visible change, and+
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remarkably high compression levels are possible if you can tolerate a+
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low-quality image.  For more details, see the references, or just experiment+
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with various compression settings.+
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+
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This software implements JPEG baseline, extended-sequential, and progressive+
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compression processes.  Provision is made for supporting all variants of these+
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processes, although some uncommon parameter settings aren't implemented yet.+
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For legal reasons, we are not distributing code for the arithmetic-coding+
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variants of JPEG; see LEGAL ISSUES.  We have made no provision for supporting+
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the hierarchical or lossless processes defined in the standard.+
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+
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We provide a set of library routines for reading and writing JPEG image files,+
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plus two sample applications "cjpeg" and "djpeg", which use the library to+
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perform conversion between JPEG and some other popular image file formats.+
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The library is intended to be reused in other applications.+
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+
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In order to support file conversion and viewing software, we have included+
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considerable functionality beyond the bare JPEG coding/decoding capability;+
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for example, the color quantization modules are not strictly part of JPEG+
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decoding, but they are essential for output to colormapped file formats or+
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colormapped displays.  These extra functions can be compiled out of the+
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library if not required for a particular application.  We have also included+
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"jpegtran", a utility for lossless transcoding between different JPEG+
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processes, and "rdjpgcom" and "wrjpgcom", two simple applications for+
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inserting and extracting textual comments in JFIF files.+
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+
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The emphasis in designing this software has been on achieving portability and+
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flexibility, while also making it fast enough to be useful.  In particular,+
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the software is not intended to be read as a tutorial on JPEG.  (See the+
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REFERENCES section for introductory material.)  Rather, it is intended to+
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be reliable, portable, industrial-strength code.  We do not claim to have+
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achieved that goal in every aspect of the software, but we strive for it.+
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+
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We welcome the use of this software as a component of commercial products.+
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No royalty is required, but we do ask for an acknowledgement in product+
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documentation, as described under LEGAL ISSUES.+
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+
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+
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LEGAL ISSUES+
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============+
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+
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In plain English:+
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+
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1. We don't promise that this software works.  (But if you find any bugs,+
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   please let us know!)+
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2. You can use this software for whatever you want.  You don't have to pay us.+
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3. You may not pretend that you wrote this software.  If you use it in a+
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   program, you must acknowledge somewhere in your documentation that+
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   you've used the IJG code.+
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+
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In legalese:+
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+
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The authors make NO WARRANTY or representation, either express or implied,+
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with respect to this software, its quality, accuracy, merchantability, or+
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fitness for a particular purpose.  This software is provided "AS IS", and you,+
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its user, assume the entire risk as to its quality and accuracy.+
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+
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This software is copyright (C) 1991-1998, Thomas G. Lane.+
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All Rights Reserved except as specified below.+
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+
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Permission is hereby granted to use, copy, modify, and distribute this+
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software (or portions thereof) for any purpose, without fee, subject to these+
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conditions:+
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(1) If any part of the source code for this software is distributed, then this+
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README file must be included, with this copyright and no-warranty notice+
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unaltered; and any additions, deletions, or changes to the original files+
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must be clearly indicated in accompanying documentation.+
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(2) If only executable code is distributed, then the accompanying+
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documentation must state that "this software is based in part on the work of+
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the Independent JPEG Group".+
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(3) Permission for use of this software is granted only if the user accepts+
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full responsibility for any undesirable consequences; the authors accept+
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NO LIABILITY for damages of any kind.+
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+
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These conditions apply to any software derived from or based on the IJG code,+
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not just to the unmodified library.  If you use our work, you ought to+
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acknowledge us.+
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+
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Permission is NOT granted for the use of any IJG author's name or company name+
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in advertising or publicity relating to this software or products derived from+
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it.  This software may be referred to only as "the Independent JPEG Group's+
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software".+
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+
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We specifically permit and encourage the use of this software as the basis of+
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commercial products, provided that all warranty or liability claims are+
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assumed by the product vendor.+
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+
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+
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ansi2knr.c is included in this distribution by permission of L. Peter Deutsch,+
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sole proprietor of its copyright holder, Aladdin Enterprises of Menlo Park, CA.+
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ansi2knr.c is NOT covered by the above copyright and conditions, but instead+
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by the usual distribution terms of the Free Software Foundation; principally,+
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that you must include source code if you redistribute it.  (See the file+
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ansi2knr.c for full details.)  However, since ansi2knr.c is not needed as part+
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of any program generated from the IJG code, this does not limit you more than+
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the foregoing paragraphs do.+
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+
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The Unix configuration script "configure" was produced with GNU Autoconf.+
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It is copyright by the Free Software Foundation but is freely distributable.+
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The same holds for its supporting scripts (config.guess, config.sub,+
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ltconfig, ltmain.sh).  Another support script, install-sh, is copyright+
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by M.I.T. but is also freely distributable.+
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+
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It appears that the arithmetic coding option of the JPEG spec is covered by+
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patents owned by IBM, AT&T, and Mitsubishi.  Hence arithmetic coding cannot+
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legally be used without obtaining one or more licenses.  For this reason,+
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support for arithmetic coding has been removed from the free JPEG software.+
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(Since arithmetic coding provides only a marginal gain over the unpatented+
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Huffman mode, it is unlikely that very many implementations will support it.)+
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So far as we are aware, there are no patent restrictions on the remaining+
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code.+
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+
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The IJG distribution formerly included code to read and write GIF files.+
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To avoid entanglement with the Unisys LZW patent, GIF reading support has+
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been removed altogether, and the GIF writer has been simplified to produce+
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"uncompressed GIFs".  This technique does not use the LZW algorithm; the+
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resulting GIF files are larger than usual, but are readable by all standard+
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GIF decoders.+
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+
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We are required to state that+
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    "The Graphics Interchange Format(c) is the Copyright property of+
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    CompuServe Incorporated.  GIF(sm) is a Service Mark property of+
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    CompuServe Incorporated."+
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+
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+
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REFERENCES+
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==========+
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+
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We highly recommend reading one or more of these references before trying to+
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understand the innards of the JPEG software.+
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+
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The best short technical introduction to the JPEG compression algorithm is+
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	Wallace, Gregory K.  "The JPEG Still Picture Compression Standard",+
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	Communications of the ACM, April 1991 (vol. 34 no. 4), pp. 30-44.+
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(Adjacent articles in that issue discuss MPEG motion picture compression,+
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applications of JPEG, and related topics.)  If you don't have the CACM issue+
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handy, a PostScript file containing a revised version of Wallace's article is+
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available at ftp://ftp.uu.net/graphics/jpeg/wallace.ps.gz.  The file (actually+
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a preprint for an article that appeared in IEEE Trans. Consumer Electronics)+
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omits the sample images that appeared in CACM, but it includes corrections+
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and some added material.  Note: the Wallace article is copyright ACM and IEEE,+
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and it may not be used for commercial purposes.+
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+
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A somewhat less technical, more leisurely introduction to JPEG can be found in+
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"The Data Compression Book" by Mark Nelson and Jean-loup Gailly, published by+
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M&T Books (New York), 2nd ed. 1996, ISBN 1-55851-434-1.  This book provides+
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good explanations and example C code for a multitude of compression methods+
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including JPEG.  It is an excellent source if you are comfortable reading C+
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code but don't know much about data compression in general.  The book's JPEG+
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sample code is far from industrial-strength, but when you are ready to look+
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at a full implementation, you've got one here...+
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+
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The best full description of JPEG is the textbook "JPEG Still Image Data+
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Compression Standard" by William B. Pennebaker and Joan L. Mitchell, published+
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by Van Nostrand Reinhold, 1993, ISBN 0-442-01272-1.  Price US$59.95, 638 pp.+
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The book includes the complete text of the ISO JPEG standards (DIS 10918-1+
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and draft DIS 10918-2).  This is by far the most complete exposition of JPEG+
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in existence, and we highly recommend it.+
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+
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The JPEG standard itself is not available electronically; you must order a+
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paper copy through ISO or ITU.  (Unless you feel a need to own a certified+
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official copy, we recommend buying the Pennebaker and Mitchell book instead;+
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it's much cheaper and includes a great deal of useful explanatory material.)+
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In the USA, copies of the standard may be ordered from ANSI Sales at (212)+
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642-4900, or from Global Engineering Documents at (800) 854-7179.  (ANSI+
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doesn't take credit card orders, but Global does.)  It's not cheap: as of+
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1992, ANSI was charging $95 for Part 1 and $47 for Part 2, plus 7%+
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shipping/handling.  The standard is divided into two parts, Part 1 being the+
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actual specification, while Part 2 covers compliance testing methods.  Part 1+
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is titled "Digital Compression and Coding of Continuous-tone Still Images,+
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Part 1: Requirements and guidelines" and has document numbers ISO/IEC IS+
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10918-1, ITU-T T.81.  Part 2 is titled "Digital Compression and Coding of+
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Continuous-tone Still Images, Part 2: Compliance testing" and has document+
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numbers ISO/IEC IS 10918-2, ITU-T T.83.+
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+
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Some extensions to the original JPEG standard are defined in JPEG Part 3,+
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a newer ISO standard numbered ISO/IEC IS 10918-3 and ITU-T T.84.  IJG+
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currently does not support any Part 3 extensions.+
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+
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The JPEG standard does not specify all details of an interchangeable file+
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format.  For the omitted details we follow the "JFIF" conventions, revision+
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1.02.  A copy of the JFIF spec is available from:+
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	Literature Department+
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	C-Cube Microsystems, Inc.+
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	1778 McCarthy Blvd.+
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	Milpitas, CA 95035+
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	phone (408) 944-6300,  fax (408) 944-6314+
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A PostScript version of this document is available by FTP at+
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ftp://ftp.uu.net/graphics/jpeg/jfif.ps.gz.  There is also a plain text+
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version at ftp://ftp.uu.net/graphics/jpeg/jfif.txt.gz, but it is missing+
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the figures.+
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+
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The TIFF 6.0 file format specification can be obtained by FTP from+
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ftp://ftp.sgi.com/graphics/tiff/TIFF6.ps.gz.  The JPEG incorporation scheme+
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found in the TIFF 6.0 spec of 3-June-92 has a number of serious problems.+
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IJG does not recommend use of the TIFF 6.0 design (TIFF Compression tag 6).+
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Instead, we recommend the JPEG design proposed by TIFF Technical Note #2+
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(Compression tag 7).  Copies of this Note can be obtained from ftp.sgi.com or+
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from ftp://ftp.uu.net/graphics/jpeg/.  It is expected that the next revision+
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of the TIFF spec will replace the 6.0 JPEG design with the Note's design.+
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Although IJG's own code does not support TIFF/JPEG, the free libtiff library+
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uses our library to implement TIFF/JPEG per the Note.  libtiff is available+
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from ftp://ftp.sgi.com/graphics/tiff/.+
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+
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+
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ARCHIVE LOCATIONS+
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=================+
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+
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The "official" archive site for this software is ftp.uu.net (Internet+
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address 192.48.96.9).  The most recent released version can always be found+
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there in directory graphics/jpeg.  This particular version will be archived+
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as ftp://ftp.uu.net/graphics/jpeg/jpegsrc.v6b.tar.gz.  If you don't have+
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direct Internet access, UUNET's archives are also available via UUCP; contact+
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help@uunet.uu.net for information on retrieving files that way.+
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+
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Numerous Internet sites maintain copies of the UUNET files.  However, only+
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ftp.uu.net is guaranteed to have the latest official version.+
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+
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You can also obtain this software in DOS-compatible "zip" archive format from+
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the SimTel archives (ftp://ftp.simtel.net/pub/simtelnet/msdos/graphics/), or+
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on CompuServe in the Graphics Support forum (GO CIS:GRAPHSUP), library 12+
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"JPEG Tools".  Again, these versions may sometimes lag behind the ftp.uu.net+
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release.+
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+
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The JPEG FAQ (Frequently Asked Questions) article is a useful source of+
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general information about JPEG.  It is updated constantly and therefore is+
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not included in this distribution.  The FAQ is posted every two weeks to+
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Usenet newsgroups comp.graphics.misc, news.answers, and other groups.+
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It is available on the World Wide Web at http://www.faqs.org/faqs/jpeg-faq/+
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and other news.answers archive sites, including the official news.answers+
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archive at rtfm.mit.edu: ftp://rtfm.mit.edu/pub/usenet/news.answers/jpeg-faq/.+
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If you don't have Web or FTP access, send e-mail to mail-server@rtfm.mit.edu+
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with body+
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	send usenet/news.answers/jpeg-faq/part1+
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	send usenet/news.answers/jpeg-faq/part2+
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+
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+
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RELATED SOFTWARE+
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================+
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+
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Numerous viewing and image manipulation programs now support JPEG.  (Quite a+
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few of them use this library to do so.)  The JPEG FAQ described above lists+
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some of the more popular free and shareware viewers, and tells where to+
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obtain them on Internet.+
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+
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If you are on a Unix machine, we highly recommend Jef Poskanzer's free+
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PBMPLUS software, which provides many useful operations on PPM-format image+
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files.  In particular, it can convert PPM images to and from a wide range of+
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other formats, thus making cjpeg/djpeg considerably more useful.  The latest+
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version is distributed by the NetPBM group, and is available from numerous+
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sites, notably ftp://wuarchive.wustl.edu/graphics/graphics/packages/NetPBM/.+
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Unfortunately PBMPLUS/NETPBM is not nearly as portable as the IJG software is;+
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you are likely to have difficulty making it work on any non-Unix machine.+
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+
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A different free JPEG implementation, written by the PVRG group at Stanford,+
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is available from ftp://havefun.stanford.edu/pub/jpeg/.  This program+
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is designed for research and experimentation rather than production use;+
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it is slower, harder to use, and less portable than the IJG code, but it+
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is easier to read and modify.  Also, the PVRG code supports lossless JPEG,+
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which we do not.  (On the other hand, it doesn't do progressive JPEG.)+
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+
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+
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FILE FORMAT WARS+
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================+
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+
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Some JPEG programs produce files that are not compatible with our library.+
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The root of the problem is that the ISO JPEG committee failed to specify a+
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concrete file format.  Some vendors "filled in the blanks" on their own,+
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creating proprietary formats that no one else could read.  (For example, none+
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of the early commercial JPEG implementations for the Macintosh were able to+
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exchange compressed files.)+
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+
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The file format we have adopted is called JFIF (see REFERENCES).  This format+
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has been agreed to by a number of major commercial JPEG vendors, and it has+
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become the de facto standard.  JFIF is a minimal or "low end" representation.+
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We recommend the use of TIFF/JPEG (TIFF revision 6.0 as modified by TIFF+
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Technical Note #2) for "high end" applications that need to record a lot of+
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additional data about an image.  TIFF/JPEG is fairly new and not yet widely+
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supported, unfortunately.+
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+
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The upcoming JPEG Part 3 standard defines a file format called SPIFF.+
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SPIFF is interoperable with JFIF, in the sense that most JFIF decoders should+
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be able to read the most common variant of SPIFF.  SPIFF has some technical+
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advantages over JFIF, but its major claim to fame is simply that it is an+
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official standard rather than an informal one.  At this point it is unclear+
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whether SPIFF will supersede JFIF or whether JFIF will remain the de-facto+
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standard.  IJG intends to support SPIFF once the standard is frozen, but we+
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have not decided whether it should become our default output format or not.+
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(In any case, our decoder will remain capable of reading JFIF indefinitely.)+
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+
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Various proprietary file formats incorporating JPEG compression also exist.+
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We have little or no sympathy for the existence of these formats.  Indeed,+
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one of the original reasons for developing this free software was to help+
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force convergence on common, open format standards for JPEG files.  Don't+
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use a proprietary file format!+
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+
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+
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TO DO+
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=====+
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+
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The major thrust for v7 will probably be improvement of visual quality.+
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The current method for scaling the quantization tables is known not to be+
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very good at low Q values.  We also intend to investigate block boundary+
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smoothing, "poor man's variable quantization", and other means of improving+
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quality-vs-file-size performance without sacrificing compatibility.+
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+
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In future versions, we are considering supporting some of the upcoming JPEG+
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Part 3 extensions --- principally, variable quantization and the SPIFF file+
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format.+
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+
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As always, speeding things up is of great interest.+
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+
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Please send bug reports, offers of help, etc. to jpeg-info@uunet.uu.net.+
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