Space & Astronomy/Space and Astronomy (October 1993).iso/mac/VIEWERS/MAC/jpeg2gif/djpeg folder/README

Text File | 1991-11-06 | 19KB | 392 lines

The Independent JPEG Group's JPEG software
==========================================

README for release of  7-Oct-91
===============================

This distribution contains the first 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.

This software is still undergoing revision.  Updated versions may be obtained
by anonymous FTP to uunet.uu.net; look under directory /graphics/jpeg.  This
particular version will be archived as jpegsrc.v1.tar.Z.  If you don't have
access to Internet FTP, UUNET's archives are also available via UUCP; contact
[e-mail] for information on retrieving files that way.

Please report any problems with this software to [e-mail].

If you intend to become a serious user of this software, please contact
jpeg-info@uunet to be added to our electronic mailing list.  Then you'll be
notified of updates and have a chance to participate in discussions, etc.

This software is the work of Tom Lane, Philip Gladstone, Luis Ortiz, 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.)


WHAT'S HERE
===========

This distribution contains software to implement JPEG image compression and
decompression.  JPEG is a standardized compression method for full-color and
gray-scale images.  JPEG is intended for "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 should 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 hardly any visible change, and amazingly high compression levels
can be obtained 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 or lossless processes defined in the standard.

The present software is still largely in the prototype stage.  It does not
support all possible variants of the JPEG standard, and some functions have
rather slow and/or crude implementations.  However, it is useful already.

The emphasis in designing this software has been on achieving portability and
flexibility, while also making it fast enough to be useful.  We have not yet
undertaken serious performance measurement or tuning; we intend to do so in
the future.


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 SETUP), compile and go.
  Members of the independent JPEG group will improve the out-of-the-box
  functionality 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 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 the basis for commercial
products; no royalty is required.


SETUP
=====

The installation process is not very automatic; you will need at least some
familiarity with C programming and program build procedures for your system.
(Volunteers to work on improving this situation are welcome.  Also, we will
probably start distributing pre-built binaries for popular systems at some
point.)

First, select a makefile and copy it to "Makefile".  "makefile.unix"
is appropriate for most Unix and Unix-like systems.  Special makefiles are
included for various PC compilers.  If you don't see a makefile for your
system, we recommend starting from makefile.unix.

Look over the Makefile and adjust options as needed.  In particular, you'll
need to change the CC= and CFLAGS= definitions if you don't have gcc
(makefile.unix only).  If you have a function-prototype-less compiler, be sure
to uncomment the .c.o rule and say "make ansi2knr".  This will cause the
source files to be preprocessed to change our ANSI-style function definitions
to old-style definitions.  (Thanks to Peter Deutsch of Aladdin Enterprises for
ansi2knr.)

Also look over jconfig.h and adjust #defines as necessary.  If you have an
ANSI-compliant C compiler (gcc for instance), no changes should be necessary
except perhaps for RIGHT_SHIFT_IS_UNSIGNED and TWO_FILE_COMMANDLINE.  For
older compilers other mods may be needed, depending on what ANSI features are
supported.  If you prefer, you can usually leave jconfig.h unmodified and add
-D switches to the Makefile's CFLAGS= definition.

Then say "make".

If you have trouble with missing system include files or inclusion of the
wrong ones, you can fix it in jinclude.h.  In particular, if you are using
gcc on a machine with non-ANSI system include files, you are likely to find
that jinclude.h tries to include the wrong files (because gcc defines
__STDC__).  There's no good automatic solution to this, so you'll just have
to hand-edit jinclude.h.

As a quick test of functionality we've included three sample files:
    testorig.jpg    same as blkint.jpg from JPEG validation floppy.
    testimg.ppm    output of djpeg testorig.jpg
    testimg.jpg    output of cjpeg testimg.ppm
The two .jpg files aren't identical due to different parameter choices (and
wouldn't be anyway, since JPEG is lossy).  However, if you can generate
duplicates of testimg.ppm and testimg.jpg then you probably have a working
port.  "make test" will perform the necessary comparisons (by generating
testout.ppm and testout.jpg and comparing these to testimg.*).  NOTE: this
is far from an exhaustive test of the JPEG software; some modules, such as
color quantization and GIF I/O, are not exercised at all.  It's just a quick
test to give you some confidence that you haven't missed something major.

If you need to make a smaller version of the JPEG software, some optional
functions can be removed at compile time.  See the xxx_SUPPORTED #defines
in jconfig.h.  (Not a lot is actually removed right now, but as more optional
stuff gets added, this mechanism will start to make a difference.)

If you want to incorporate the JPEG code as subroutines in a larger program,
we recommend that you make libjpeg.a.  Then use the .h files and libjpeg.a as
your interface to the JPEG functions.  Your surrounding program will have to
provide functionality similar to what's in jcmain.c or jdmain.c, and you may
want to replace jerror.c and possibly other modules depending on your needs.
See the "architecture" file for more info.  If it seems to you that the system
structure doesn't accommodate what you want to do, please contact the authors.

Special notes for Macintosh Think C users: If you have version 5.0 you should
be able to just turn on __STDC__ through the compiler switch that enables
that.  With version 4.0 you must manually edit jconfig.h to define PROTO,
HAVE_UNSIGNED_CHAR, HAVE_UNSIGNED_SHORT, and const.  (It seems to be safe to
just define __STDC__ to take care of the first three.)  When setting up
project files, use the COBJECTS and DOBJECTS lists in makefile.unix as a guide
to which files need to be included, and add the ANSI and Unix C libraries in a
separate segment.  You may need to divide the JPEG files into more than one
segment; you can do this pretty much as you please.


USAGE
=====

The user interface is pretty minimal at this point.  We haven't bothered to
generate manual-page files since the switches badly need redesign.  At the
moment, things work like this:

There are two programs, cjpeg to compress an image file into JPEG format,
and djpeg to decompress.

On Unix systems, you say:
    cjpeg [switches] [imagefile] >jpegfile
    djpeg [switches] [jpegfile]  >imagefile
The programs read the specified input file, or standard input if none is
named.  They always write to standard output (with trace/error messages to
standard error).  These conventions are handy for piping images between
programs.

On PC, Macintosh, and Amiga systems, you say:
    cjpeg [switches] imagefile jpegfile
    djpeg [switches] jpegfile  imagefile
i.e., both input and output files are named on the command line.  This style
is a little more foolproof, and it loses no functionality if you don't have
pipes.  You can get this style on Unix too, if you prefer, by defining
TWO_FILE_COMMANDLINE in jconfig.h or in the Makefile.  You MUST use this style
on any system that doesn't cope well with binary data fed through
stdin/stdout.

Currently supported image file formats include raw-format PPM, raw-format PGM
(for monochrome images), and GIF.  cjpeg recognizes the input image format
automatically, but you have to tell djpeg which format to generate.

The only JPEG file format currently supported is a raw JPEG data stream.
Unless modified, the programs use the JFIF conventions for variables left
unspecified by the JPEG standard.  (In particular, cjpeg generates a JFIF APP0
marker.)  Support for the JPEG-in-TIFF format will probably be added at some
future date.

The command line switches for cjpeg are:

    -I        Generate noninterleaved JPEG file (not yet supported).

    -Q quality    Scale quantization tables to adjust quality.
            Quality is 0 (worst) to 100 (best); default is 75.
            (See below for more info.)

    -a        Use arithmetic coding rather than Huffman coding.
            (Not currently supported, see LEGAL ISSUES.)

    -o        Perform optimization of entropy encoding parameters.
            Without this, default Huffman or arithmetic
            parameters are used.  -o makes the JPEG file a tad
            smaller, but compression uses much more memory.
            Image quality is unaffected by -o.

    -d        Enable debug printout.  More -d's give more printout.

Typically you'd use -Q settings of 50 or 75 or so.  -Q 100 will generate a
quantization table of all 1's, meaning no quantization loss; then any
differences between input and output images are due to subsampling or to
roundoff error in the DCT or colorspace-conversion steps.  -Q values below 50
may be useful for making real small, low-quality images.  Try -Q 2 (or so) for
some amusing Cubist effects.  (Note that -Q values below about 25 generate
2-byte quantization tables, which are not decodable by pure baseline JPEG
decoders.  cjpeg emits a warning message when you give such a -Q value.)

The command line switches for djpeg are:

    -G        Select GIF output format (implies -q, with default
            of 256 colors).

    -b        Perform cross-block smoothing.  This is quite
            memory-intensive and only seems to improve the image
            at very low quality settings (-Q 10 to 20 or so).

    -g        Force gray-scale output even if input is color.

    -q N        Quantize to N colors.

    -D        Use Floyd-Steinberg dithering in color quantization.

    -2        Use two-pass color quantization (not yet supported).

    -d        Enable debug printout.  More -d's give more printout.

Color quantization currently uses a rather shoddy algorithm (although it's not
so horrible when dithered).  Because of this, the GIF output mode is not
recommended in the current release, except for gray-scale output.  You can get
better results by applying ppmquant to the unquantized (PPM) output of djpeg,
then converting to GIF with ppmtogif.  We expect to provide a considerably
better quantization algorithm in a future release.

Note that djpeg *can* read noninterleaved JPEG files even though cjpeg can't
yet generate them.  For most applications this is a nonissue, since hardly
anybody seems to be using noninterleaved format.

On a non-virtual-memory machine, you may run out of memory if you use -I or -o
in cjpeg, or -q ... -2 in djpeg, or try to read an interlaced GIF file.  This
will be addressed eventually by replacing jvirtmem.c with something that uses
temporary files for large images (see TO DO).


REFERENCES
==========

The best and most readily available introduction to the JPEG compression
algorithm is Wallace's article in the April '91 CACM:
    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.)  We highly recommend reading that
article before looking at any of the JPEG software.

For more detail about the JPEG standard you pretty much have to go to the
draft standard, which is not nearly as intelligible as Wallace's article.
The current version is ISO/IEC Committee Draft CD 10918-1 dated 1991-03-15.
The standard is not presently available electronically; you must order a paper
copy through ISO.

The JPEG standard does not specify all details of an interchangeable file
format.  For the omitted details we follow the "JFIF" conventions, revision
1.01.  A copy of the JFIF spec is available from:
    Literature Department
    C-Cube Microsystems, Inc.
    399A West Trimble Road
    San Jose, CA  95131
    [phone]
Requests can also be e-mailed to [e-mail] (this address good after
10/10/91).  The same source can supply copies of the draft JPEG-in-TIFF specs.

If you want to understand this implementation, start by reading the
"architecture" documentation file.  Please read "codingrules" if you want to
contribute any code.


SUPPORTING SOFTWARE
===================

You will probably want Jef Poskanzer's PBMPLUS image software; this 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).

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.


LEGAL ISSUES
============

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, 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.

Permission is NOT granted for the use of any author's name or author's 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 product generated from the JPEG code, this does not limit you more than
the foregoing paragraphs do.


It appears that the arithmetic coding option of the JPEG spec is covered by
patents held by IBM, and possibly also patents of 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
people will choose to use it.  If you do obtain such a license, contact
[e-mail] for a copy of our arithmetic coding modules.)  So far
as we are aware, there are no patent restrictions on the remaining code.


TO DO
=====

Many of the modules need fleshing out to provide more complete
implementations, or to provide faster paths for common cases.  The greatest
needs are for (a) decent color quantization, and (b) a memory manager
implementation that can work in limited memory by swapping "big" images to
temporary files.  I (Tom Lane) am going to work on color quantization next.
Volunteers to write a PC memory manager, or to work on any other modules, are
welcome.

We'd appreciate it if people would compile and check out the code on as wide a
variety of systems as possible, and report any portability problems
encountered (with solutions, if possible).  Checks of file compatibility with
other JPEG implementations would also be of interest.  Finally, we would
appreciate code profiles showing where the most time is spent, especially on
unusual systems.

Please send bug reports, offers of help, etc. to [e-mail].