Archive Magazine 1996/ARCHIVE_96.iso/discs/utilities/utility_02/convrters2/!ChangeFSI/JPEGinfo

Text File | 1996-03-11 | 21KB | 421 lines

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

README for release 3 of  17-Mar-92
==================================

This distribution contains the third official 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 usage instructions, see section USAGE.

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.

If you intend to become a serious user of this software, please contact
[e-mail] 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,
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.)

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


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

This distribution contains compiled 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
"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 no 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, hierarchical, or lossless processes defined in the standard.

The present software is not far beyond 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.

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 (including the source) 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.v3.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
[e-mail] for information on retrieving files that way.

Various other Internet sites maintain copies of the UUNET file, which may or
may not be up-to-date.  In Europe, try nic.funet.fi (128.214.6.100; look in
directory pub/graphics/programs/jpeg).

You can also obtain this software from CompuServe, in the GRAPHSUPPORT forum
(GO PICS), library 10; this version will be file jpsrc3.zip.


SOFTWARE THAT'S NO HELP AT ALL
==============================

Handmade Software's shareware PC program GIF2JPG produces files that are
totally incompatible with our programs.  They use a proprietary format that is
an amalgam of GIF and JPEG representations.  However, you can force GIF2JPG
to produce compatible files with its -j switch, and their decompression
program JPG2GIF can read our files (at least ones produced with our default
option settings).

Unfortunately, many commercial JPEG implementations are also incompatible as
of this writing, especially programs released before summer 1991.  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 we expect
that it will become the de facto standard.  JFIF is a minimal representation;
work is also going forward to incorporate JPEG compression into the TIFF
standard, for use in "high end" applications that need to record a lot of
additional data about an image.  We intend to support JPEG-in-TIFF in the
future.  We hope that these two formats will be sufficient and that other,
incompatible JPEG file formats will not proliferate.

Indeed, part of the reason for developing and releasing this free software is
to help force rapid convergence to de facto standards for JPEG file formats.
SUPPORT STANDARD, NON-PROPRIETARY FORMATS: demand JFIF or JPEG-in-TIFF!


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 trying to understand the innards of any JPEG software.
If you don't have the CACM issue handy, a PostScript file containing a revised
version of the article is available at ftp.uu.net, graphics/jpeg/wallace.ps.Z.
The file (actually a preprint for an article to appear 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.

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 standard is not now available electronically; you must order a paper copy
through ISO.  In the US, copies may be ordered from ANSI Sales at (212)
642-4900.  The standard is divided into two parts: Part 1 is the actual
specification, and Part 2 covers compliance testing methods.  The current
"committee draft" version of Part 1 is titled "Digital Compression and Coding
of Continuous-tone Still Images, Part 1: Requirements and guidelines" and has
document number ISO/IEC CD 10918-1.  (The alternate number SC2 N2215 should
also be mentioned when ordering.)  This draft is expected to be superseded by
the Draft International Standard version around the end of November 1991.
Ordering info will be the same as above, but replace "CD" with "DIS" in the
document number (alternate number not yet known).  The committee draft of
Part 2 is expected to be available around the end of December 1991.  It will
be titled "Digital Compression and Coding of Continuous-tone Still Images,
Part 2: Compliance testing" and will have document number ISO/IEC CD 10918-2
(alternate number not yet known).

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]
The same source can supply copies of the draft JPEG-in-TIFF documents
(Appendixes O and P to the TIFF spec).  PostScript versions of these
documents can also be obtained by e-mail from the C-Cube mail server,
[e-mail].  Send the message "send jfif_ps from jpeg" to obtain the
JFIF document; "send app_o_ps from jpeg" and "send app_p_ps from jpeg" will
produce the TIFF documents.  Send the message "help" if you have trouble.


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, 1992, 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.

It appears that the arithmetic coding option of the JPEG spec is covered by
patents owned by IBM and AT&T, as well as a pending Japanese patent of
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 the necessary licenses,
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.


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."


TO DO
=====

Many of the modules need fleshing out to provide more complete
implementations, or to provide faster paths for common cases.
Improving the speed will be the next big work item for the JPEG group.

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].

USAGE instructions for the Independent JPEG Group's JPEG software
=================================================================

GENERAL USAGE

We provide two programs, cjpeg to compress an image file into JPEG format,
and djpeg to decompress a JPEG file back into a conventional image format.

On Unix-like systems, you say:
        cjpeg [switches] [imagefile] >jpegfile
or
        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 most non-Unix systems, you say:
        cjpeg [switches] imagefile jpegfile
or
        djpeg [switches] jpegfile  imagefile
i.e., both the 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 when you compile the programs; see SETUP.)

The currently supported image file formats are: PPM (PBMPLUS color format),
PGM (PBMPLUS gray-scale format), GIF and Targa. cjpeg recognizes the input
image format automatically, with the exception of some Targa-format files.  You
have to tell djpeg which format to generate (PPM by default).

The only JPEG file format currently supported is the JFIF format.  Support for
the TIFF/JPEG format will probably be added at some future date.


CJPEG DETAILS

The command line switches for cjpeg are:

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

        -o              Perform optimization of entropy encoding parameters.
                        Without this, default encoding parameters are used.
                        -o usually makes the JPEG file a little smaller, but
                        cjpeg runs somewhat slower and needs much more memory.
                        Image quality and speed of decompression are unaffected
                        by -o.

        -T              Input file is Targa format.  Targa files that contain
                        an "identification" field will not be automatically
                        recognized by cjpeg; for such files you must specify
                        -T to force cjpeg to treat the input as Targa format.

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

        -a              Use arithmetic coding rather than Huffman coding.
                        (Not currently supported for legal reasons.)

        -d              Enable debug printout.  More -d's give more printout.
                        Also, version information is printed at startup.

        -m memory       Set limit for amount of memory to use in processing
                        large images.  Value is in thousands of bytes, or
                        millions of bytes if "M" is attached to the number.
                        For example, -m 4m selects 4000000 bytes.  If more
                        space is needed, temporary files will be used.

The -Q switch lets you trade off compressed file size against quality of the
reconstructed image: the higher the -Q setting, the larger the JPEG file, and
the closer the output image will be to the original input.  Normally you want
to use the lowest -Q setting (smallest file) that decompresses into something
visually indistinguishable from the original image.  For this purpose the -Q
setting should be between 50 and 95; the default of 75 is often about right.
If you see defects at -Q 75, then go up 5 or 10 counts at a time until you are
happy with the output image.  (The optimal setting will vary from one image to
another.)

-Q 100 will generate a quantization table of all 1's, eliminating loss in the
quantization step (but there is still information loss in subsampling, as well
as roundoff error).  This setting is mainly of interest for experimental
purposes.  -Q values above about 95 are NOT recommended for normal use; the
compressed file size goes up dramatically for hardly any gain in output image
quality.

In the other direction, -Q values below 50 will produce very small files of
low image quality.  Settings around 5 to 10 might be useful in preparing an
index of a large image library, for example.  Try -Q 2 (or so) for some
amusing Cubist effects.  (Note: -Q values below about 25 generate 2-byte
quantization tables, which are considered optional in the JPEG standard.
cjpeg emits a warning message when you give such a -Q value, because some
commercial JPEG programs may be unable to decode the resulting file.)


DJPEG DETAILS

The command line switches for djpeg are:

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

        -P              Select PPM or PGM output format (this is the default).
                        PGM is emitted if the JPEG file is gray-scale or if -g
                        is specified.

        -R              Select RLE output format.  Requires URT library.

        -T              Select Targa output format.  Gray-scale format is
                        emitted if the JPEG file is gray-scale or if -g is
                        specified; otherwise, colormapped format is emitted
                        if -q is specified; otherwise, 24-bit full-color
                        format is emitted.

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

        -q N            Quantize to N colors.  This reduces the number of
                        colors in the output image so that it can be displayed
                        on a colormapped display or stored in a colormapped
                        file format.  For example, if you have an 8-bit
                        display, you'd need to quantize to 256 or fewer colors.

        -D              Do not use dithering in color quantization.
                        By default, Floyd-Steinberg dithering is applied when
                        quantizing colors, but on some images dithering may
                        result in objectionable "graininess".  If that
                        happens, you can turn off dithering with -D.
                        -D is ignored unless you also say -q or -G.

        -1              Use one-pass instead of two-pass color quantization.
                        The one-pass method is faster and needs less memory,
                        but it produces a lower-quality image.
                        -1 is ignored unless you also say -q or -G.  Also,
                        the one-pass method is always used for gray-scale
                        output (the two-pass method is no improvement then).

        -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).
                        At normal -Q settings it may make the image worse.

        -d              Enable debug printout.  More -d's give more printout.
                        Also, version information is printed at startup.

        -m memory       Set limit for amount of memory to use in processing
                        large images.  Value is in thousands of bytes, or
                        millions of bytes if "M" is attached to the number.
                        For example, -m 4m selects 4000000 bytes.  If more
                        space is needed, temporary files will be used.


HINTS

Avoid running an image through a series of JPEG compression/decompression
cycles.  Image quality loss will accumulate; after ten or so cycles the image
may be noticeably worse than it was after one cycle.  It's best to use a
lossless format while manipulating an image, then convert to JPEG format when
you are ready to file the image away.

The -o option to cjpeg is worth using when you are making a "final" version
for posting or archiving.  It's also a win when you are using low -Q settings
to make very small JPEG files; the percentage improvement is often a lot more
than it is on larger files.

The default memory usage limit (-m) is set when the software is compiled.
If you get an "insufficient memory" error, try specifying a smaller -m value,
even -m 0 to use the absolute minimum space.  You may want to recompile with
a smaller default value if this happens often.

djpeg with two-pass color quantization requires a good deal of space; on
MS-DOS machines it may run out of memory even with -m 0.  In that case you
can still decompress, with some loss of image quality, by specifying -1
for one-pass quantization.

If more space is needed than will fit in the available main memory (as
determined by -m), temporary files will be used.  The temporary files are often
rather large: in typical cases they occupy three bytes per pixel, for example
3*800*600 = 1.44Mb for an 800x600 image.  If you don't have enough free disk
space, leave out -o (for cjpeg) or specify -1 (for djpeg).