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libjpg
The Independent JPEG Group’s JPEG software
README for release 6b of 27-Mar-1998
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 [email protected] to be added to our
electronic mailing list. Mailing list members are notified of updates and have a
chance to participate in technical discussions, etc.
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 Unix-style man pages for programs (same info as usage.doc).
wizard.doc
Advanced usage instructions for JPEG wizards only.
change.log
Version-to-version change highlights.
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 if you contribute code.
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 “jay-peg”) is a standardized compression
method for full-color and gray-scale images. JPEG is intended for compressing
real-world scenes; line drawings, cartoons and other non-realistic images are not
its strong suit. JPEG is lossy, meaning that the output image is not exactly identical
to the input image. Hence you must not use JPEG if you have to have identical
output bits. However, on typical photographic images, very good compression
levels can be obtained with no visible change, and remarkably high compression
levels are possible if you can tolerate a low-quality image. For more details, see the
references, or just experiment with various compression settings.
This software implements JPEG baseline, extended-sequential, and progressive
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 variants of
JPEG; see LEGAL ISSUES. We have made no provision for supporting the
hierarchical or lossless processes defined in the standard.
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 © 1991-1998, 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.
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, install-sh, is copyright by M.I.T. but is also freely
distributable.
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. 30-44.
(Adjacent articles in that issue discuss MPEG motion picture compression,
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
