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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.
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
LICENSE
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.
Useful
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
A PostScript version of this document is available by FTP at ftp://ftp.
uu.net/graphics/jpeg/jfif.ps.gz. There is also a plain text version at
ftp://ftp.uu.net/graphics/jpeg/jfif.txt.gz, but it is missing the figures.
The TIFF 6.0 file format specification can be obtained by FTP from
ftp://ftp.sgi.com/graphics/tiff/TIFF6.ps.gz.
The JPEG incorporation
scheme found in the TIFF 6.0 spec of 3-June-92 has a number of
serious problems.
IJG does not recommend use of the TIFF 6.0 design (TIFF
Compression tag 6).
Instead, we recommend the JPEG design proposed by TIFF Technical
Note #2 (Compression tag 7). Copies of this Note can be obtained
from ftp.sgi.com or from ftp://ftp.uu.net/graphics/jpeg/. It is expected
that the next revision of the TIFF spec will replace the 6.0 JPEG
design with the Note's design.
Although IJG's own code does not support TIFF/JPEG, the free libtiff
library uses our library to implement TIFF/JPEG per the Note. libtiff
is available from ftp://ftp.sgi.com/graphics/tiff/.
ARCHIVE LOCATIONS
The "official" archive site for this software is ftp.uu.net (Internet
address 192.48.96.9). The most recent released version can always
be found there in directory graphics/jpeg. This particular version will
be archived as ftp://ftp.uu.net/graphics/jpeg/jpegsrc.v6b.tar.gz.
If you don't have direct Internet access, UUNET's archives are also
available via UUCP; contact help@uunet.uu.net for information on
retrieving files that way.
Numerous Internet sites maintain copies of the UUNET files.
However, only ftp.uu.net is guaranteed to have the latest official
version.
You can also obtain this software in DOS-compatible "zip" archive
format from the SimTel archives (ftp://ftp.simtel.net/pub/simtelnet/
msdos/graphics/), or on CompuServe in the Graphics Support forum
(GO CIS:GRAPHSUP), library 12 "JPEG Tools" . Again, these versions
may sometimes lag behind the ftp.uu.net release.
The JPEG FAQ (Frequently Asked Questions) article is a useful
source of general information about JPEG. It is updated constantly
and therefore is not included in this distribution. The FAQ is posted
every two weeks to Usenet newsgroups comp.graphics.misc, news.
answers, and other groups.
It is available on the World Wide Web at http://www.faqs.org/faqs/
jpeg-faq/ and other news.answers archive sites, including the official
news.answers archive at rtfm.mit.edu: ftp://rtfm.mit.edu/pub/usenet/
news.answers/jpeg-faq/.
If you don't have Web or FTP access, send e-mail to mail-server@
rtfm.mit.edu with body
send usenet/news.answers/jpeg-faq/part1
send usenet/news.answers/jpeg-faq/part2
RELATED SOFTWARE
Numerous viewing and image manipulation programs now support
JPEG. (Quite a few of them use this library to do so.) The JPEG FAQ
described above lists some of the more popular free and shareware
viewers, and tells where to obtain them on Internet.
If you are on a Unix machine, we highly recommend Jef Poskanzer's
free PBMPLUS software, which 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, thus making cjpeg/djpeg
considerably more useful. The latest version is distributed by the
NetPBM group, and is available from numerous sites, notably ftp://
wuarchive.wustl.edu/graphics/graphics/packages/NetPBM/.
Unfortunately PBMPLUS/NETPBM is not nearly as portable as the
IJG software is; you are likely to have difficulty making it work on any
non-Unix machine.
A different free JPEG implementation, written by the PVRG group
at Stanford,
is available from ftp://havefun.stanford.edu/pub/jpeg/. This program
is designed for research and experimentation rather than production
use; it is slower, harder to use, and less portable than the IJG code,
but it is easier to read and modify. Also, the PVRG code supports
lossless JPEG, which we do not. (On the other hand, it doesn't do
progressive JPEG.)
FILE FORMAT WARS
Some JPEG programs produce files that are not compatible with our
library.
The root of the problem is that the ISO JPEG committee failed to
specify a concrete file format. Some vendors "filled in the blanks" on
their own, creating proprietary formats that no one else could read.
(For example, none of the early commercial JPEG implementations
for the Macintosh were able to exchange compressed files.)
The file format we have adopted is called JFIF (see REFERENCES).
This format has been agreed to by a number of major commercial
JPEG vendors, and it has become the de facto standard. JFIF is a
minimal or "low end" representation.
We recommend the use of TIFF/JPEG (TIFF revision 6.0 as modified
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