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The Independent JPEG Group's JPEG software
==========================================
README for release 4 of 10-Dec-92
=================================
This distribution contains the fourth 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.
For installation instructions, see file SETUP.
For usage instructions, see file USAGE (or the cjpeg.1 and djpeg.1 manual
pages; but USAGE contains a "hints" section not found in the manual pages).
Useful information can also be found in the JPEG FAQ (Frequently Asked
Questions) article; see ARCHIVE LOCATIONS below to obtain the FAQ article.
This software is still undergoing revision. Updated versions may be obtained
by FTP or UUCP to UUNET and other archive sites; see ARCHIVE LOCATIONS below
for details.
Serious users of this software (particularly those incorporating it into
larger programs) should contact jpeg-info@uunet.uu.net 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, Luis Ortiz,
Lee Crocker, Ge' Weijers, and other members of the Independent JPEG Group.
DISCLAIMER
==========
THIS SOFTWARE IS NOT COMPLETE NOR FULLY DEBUGGED. It is not guaranteed to be
useful for anything, nor to be compatible with subsequent releases, nor to be
an accurate implementation of the JPEG standard. (See LEGAL ISSUES for even
more disclaimers.)
Despite that, we believe that this software is pretty good, and if you find
any problems with it, we'd like to know about them. Please report problems
by e-mail to jpeg-info@uunet.uu.net.
WHAT'S HERE
===========
This distribution 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; cartoons and other non-realistic images are not its
strong suit. JPEG is lossy, meaning that the output image is not necessarily
identical to the input image. Hence you must not use JPEG if you have to have
identical output bits. However, on typical images of real-world scenes, very
good compression levels can be obtained with no visible change, and amazingly
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.
The software implements JPEG baseline and extended-sequential 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 process; see
LEGAL ISSUES. At present we have made no provision for supporting the
progressive, hierarchical, or lossless processes defined in the standard.
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 if not
required for a particular application.
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.) While we hope that the entire
package will someday be industrial-strength code, much remains to be done in
performance tuning and in improving the capabilities of individual modules.
This software can be used on several levels:
* As canned software for JPEG compression and decompression. Just edit the
Makefile and configuration files as needed (see file SETUP), compile and go.
Members of the Independent JPEG Group will improve the out-of-the-box
functionality and speed as time goes on.
* As the basis for other JPEG programs. For example, you could incorporate
the decompressor into a general image viewing package by replacing the
output module with write-to-screen functions. For an implementation on
specific hardware, you might want to replace some of the inner loops with
assembly code. For a non-command-line-driven system, you might want a
different user interface. (Members of the group will be producing Macintosh
and Amiga versions with more appropriate user interfaces, for example.)
* As a toolkit for experimentation with JPEG and JPEG-like algorithms. Most
of the individual decisions you might want to mess with are packaged up into
separate modules. For example, the details of color-space conversion and
subsampling techniques are each localized in one compressor and one
decompressor module. You'd probably also want to extend the user interface
to give you more detailed control over the JPEG compression parameters.
In particular, we welcome the use of this software as a component of commercial
products; no royalty is required.
ARCHIVE LOCATIONS
=================
The "official" archive site for this software is ftp.uu.net (Internet
address 137.39.1.9 or 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 jpegsrc.v4.tar.Z. If you are on the Internet, you can
retrieve files from UUNET by anonymous FTP. If you don't have FTP access,
UUNET's archives are also available via UUCP; contact postmaster@uunet.uu.net
for information on retrieving files that way.
Numerous Internet sites maintain copies of the UUNET files; in particular,
you can probably find a copy at any site that archives comp.sources.misc
submissions. However, only ftp.uu.net is guaranteed to have the latest
official version.
You can also obtain this software from CompuServe, in the GRAPHSUPPORT forum
(GO PICS), library 15; this version will be file jpsrc4.zip. Again,
CompuServe is not guaranteed to have the very latest version.
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, news.answers, and other groups. You
can always obtain the latest version from the news.answers archive at
rtfm.mit.edu (18.172.1.27). By FTP, fetch /pub/usenet/news.answers/jpeg-faq.
If you don't have FTP, send e-mail to mail-server@rtfm.mit.edu with body
"send usenet/news.answers/jpeg-faq".
SUPPORTING SOFTWARE
===================
You will probably want Jef Poskanzer's PBMPLUS image 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. You can FTP
this free software from export.lcs.mit.edu (contrib/pbmplus*.tar.Z) or
ftp.ee.lbl.gov (pbmplus*.tar.Z). Unfortunately PBMPLUS is not nearly as
portable as the JPEG software is; you are likely to have difficulty making it
work on any non-Unix machine.
If you are using X Windows you might want to use the xv or xloadimage viewers
to save yourself the trouble of converting PPM to some other format. Both of
these can be found in the contrib directory at export.lcs.mit.edu. Actually,
xv version 2.00 and up incorporates our software and thus can read and write
JPEG files directly. (NOTE: since xv internally reduces all images to 8
bits/pixel, a JPEG file written by xv will not be very high quality; and xv
cannot fully exploit a 24-bit display. These problems are expected to go away
in the next xv release, planned for early 1993. In the meantime, use
xloadimage for 24-bit displays.)
For DOS machines, Lee Crocker's free Piclab program is a useful companion to
the JPEG software. The latest version, currently 1.91, is available