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LICENSE
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 (C) 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 Wallace
article is copyright ACM and IEEE, and it may not be used
for commercial purposes.
A somewhat less technical, more leisurely introduction
to JPEG can be found in “The Data Compression Book”
by Mark Nelson and Jean-loup Gailly, published by M & T
Books (New York), 2nd ed. 1996, ISBN 1-55851-434-1. This
book provides good explanations and example C code for
a multitude of compression methods including JPEG. It
is an excellent source if you are comfortable reading C
code but don’t know much about data compression
in general. The book’s JPEG sample code is far from
industrial-strength, but when you are ready to look at a full
implementation, you’ve got one here...
The best full description of JPEG is the textbook “JPEG
Still Image Data Compression Standard” by William
B. Pennebaker and Joan L. Mitchell, published by Van
Nostrand Reinhold, 1993, ISBN 0-442-01272-1. Price
US$59.95, 638 pp.
The book includes the complete text of the ISO JPEG
standards (DIS 10918-1 and draft DIS 10918-2).
This is by far the most complete exposition of JPEG in
existence, and we highly recommend it.
The JPEG standard itself is not available electronically;
you must order a paper copy through ISO or ITU. (Unless
you feel a need to own a certified official copy, we
recommend buying the Pennebaker and Mitchell book
instead; it’s much cheaper and includes a great deal of
useful explanatory material.)
In the USA, copies of the standard may be ordered from
ANSI Sales at (212) 642-4900, or from Global Engineering
Documents at (800) 854-7179. (ANSI doesn’t take credit
card orders, but Global does.) It’s not cheap: as of 1992,
ANSI was charging $95 for Part 1 and $47 for Part 2, plus
7% shipping/handling. The standard is divided into two
parts, Part 1 being the actual specification, while Part 2
covers compliance testing methods. Part 1 is titled “Digital
Compression and Coding of Continuous-tone Still Images,
Part 1: Requirements and guidelines” and has document
numbers ISO/IEC IS 10918-1, ITU-T T.81. Part 2 is titled
“Digital Compression and Coding of Continuous-tone Still
Images, Part 2: Compliance testing” and has document
numbers ISO/IEC IS 10918-2, ITU-T T.83.
Some extensions to the original JPEG standard are defined
in JPEG Part 3, a newer ISO standard numbered ISO/IEC
IS 10918-3 and ITU-T T.84. IJG currently does not support
any Part 3 extensions.
The JPEG standard does not specify all details of an
interchangeable file format. For the omitted details we
follow the “JFIF” conventions, revision 1.02. A copy of the
JFIF spec is available from:
Literature Department
C-Cube Microsystems, Inc.
1778 McCarthy Blvd.
Milpitas, CA 95035
phone (408) 944-6300, fax (408) 944-6314