readme

Jpeg source code,jpeg编码解码示范

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.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, published by M&T Books (Redwood
City, CA), 1991, ISBN 1-55851-216-0.  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 [phone].  (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.

Extensions to the original JPEG standard are defined in JPEG Part 3, a new ISO
document.  Part 3 is undergoing ISO balloting and is expected to be approved
by the end of 1995; it will have document numbers ISO/IEC IS 10918-3, 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 [phone],  fax [phone]
A PostScript version of this document is available at ftp.uu.net, file
graphics/jpeg/jfif.ps.gz.  It can also be obtained by e-mail from the C-Cube
mail server, [e-mail].  Send the message "send jfif_ps from jpeg"
to the server to obtain the JFIF document; send the message "help" if you have
trouble.

The TIFF 6.0 file format specification can be obtained by FTP from sgi.com
(192.48.153.1), file graphics/tiff/TIFF6.ps.Z; or you can order a printed
copy from Aldus Corp. at [phone].  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 sgi.com or
from 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 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 graphics/jpeg/jpegsrc.v6a.tar.gz.  If you are on the Internet, you
can retrieve files from ftp.uu.net by standard 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.

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.coast.net:/SimTel/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.
You can always obtain the latest version from the news.answers archive at
rtfm.mit.edu.  By FTP, fetch /pub/usenet/news.answers/jpeg-faq/part1 and
.../part2.  If you don't have FTP, send e-mail to [e-mail]
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 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 obtain this package by FTP from ftp.x.org
(contrib/pbmplus*.tar.Z) or ftp.ee.lbl.gov (pbmplus*.tar.Z).  There is also
a newer update of this package called NETPBM, available from
wuarchive.wustl.edu under directory /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 havefun.stanford.edu in directory 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.


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 by TIFF
Technical Note #2) for "high end" applications that need to record a lot of
additional data about an image.  TIFF/JPEG is fairly new and not yet widely
supported, unfortunately.

The upcoming JPEG Part 3 standard defines a file format called SPIFF.
SPIFF is interoperable with JFIF, in the sense that most JFIF decoders should
be able to read the most common variant of SPIFF.  SPIFF has some technical
advantages over JFIF, but its major claim to fame is simply that it is an
official standard rather than an informal one.  At this point it is unclear
whether SPIFF will supersede JFIF or whether JFIF will remain the de-facto
standard.  IJG intends to support SPIFF once the standard is frozen, but we
have not decided whether it should become our default output format or not.
(In any case, our decoder will remain capable of reading JFIF indefinitely.)

Various proprietary file formats incorporating JPEG compression also exist.
We have little or no sympathy for the existence of these formats.  Indeed,
one of the original reasons for developing this free software was to help
force convergence on common, open format standards for JPEG files.  Don't
use a proprietary file format!


TO DO
=====

In future versions, we are considering supporting some of the upcoming JPEG
Part 3 extensions --- principally, variable quantization and the SPIFF file
format.

Tuning the software for better behavior at low quality/high compression
settings is also of interest.  The current method for scaling the
quantization tables is known not to be very good at low Q values.

As always, speeding things up is high on our priority list.

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