304 lines
15 KiB
Plaintext
304 lines
15 KiB
Plaintext
The Independent JPEG Group's JPEG software
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==========================================
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README for release 2 of 13-Dec-91
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=================================
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This distribution contains the second public release of the Independent JPEG
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Group's free JPEG software. You are welcome to redistribute this software and
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to use it for any purpose, subject to the conditions under LEGAL ISSUES, below.
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For installation instructions, see file SETUP; for usage instructions, see
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file USAGE (or the cjpeg.1 and djpeg.1 manual pages).
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This software is still undergoing revision. Updated versions may be obtained
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by FTP or UUCP to uunet.uu.net and other archive sites; see ARCHIVE LOCATIONS
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below for details.
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If you intend to become a serious user of this software, please contact
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jpeg-info@uunet.uu.net to be added to our electronic mailing list. Then
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you'll be notified of updates and have a chance to participate in discussions,
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etc.
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This software is the work of Tom Lane, Philip Gladstone, Luis Ortiz,
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Lee Crocker, and other members of the Independent JPEG Group.
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DISCLAIMER
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==========
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THIS SOFTWARE IS NOT COMPLETE NOR FULLY DEBUGGED. It is not guaranteed to be
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useful for anything, nor to be compatible with subsequent releases, nor to be
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an accurate implementation of the JPEG standard. (See LEGAL ISSUES for even
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more disclaimers.)
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Please report any problems with this software to jpeg-info@uunet.uu.net.
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WHAT'S HERE
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===========
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This distribution contains software to implement JPEG image compression and
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decompression. JPEG (pronounced "jay-peg") is a standardized compression
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method for full-color and gray-scale images. JPEG is intended for
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"real-world" scenes; cartoons and other non-realistic images are not its
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strong suit. JPEG is lossy, meaning that the output image is not necessarily
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identical to the input image. Hence you should not use JPEG if you have to
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have identical output bits. However, on typical images of real-world scenes,
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very good compression levels can be obtained with no visible change, and
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amazingly high compression levels can be obtained if you can tolerate a
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low-quality image. For more details, see the references, or just experiment
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with various compression settings.
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The software implements JPEG baseline and extended-sequential compression
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processes. Provision is made for supporting all variants of these processes,
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although some uncommon parameter settings aren't implemented yet. For legal
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reasons, we are not distributing code for the arithmetic-coding process; see
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LEGAL ISSUES. At present we have made no provision for supporting the
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progressive or lossless processes defined in the standard.
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The present software is still largely in the prototype stage. It does not
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support all possible variants of the JPEG standard, and some functions have
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rather slow and/or crude implementations. However, it is useful already.
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The emphasis in designing this software has been on achieving portability and
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flexibility, while also making it fast enough to be useful. We have not yet
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undertaken serious performance measurement or tuning; we intend to do so in
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the future.
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This software can be used on several levels:
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* As canned software for JPEG compression and decompression. Just edit the
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Makefile and configuration files as needed (see file SETUP), compile and go.
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Members of the independent JPEG group will improve the out-of-the-box
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functionality and speed as time goes on.
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* As the basis for other JPEG programs. For example, you could incorporate
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the decompressor into a general image viewing package by replacing the
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output module with write-to-screen functions. For an implementation on
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specific hardware, you might want to replace some of the inner loops with
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assembly code. For a non-command-line-driven system, you might want a
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different user interface. (Members of the group will be producing Macintosh
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and Amiga versions with more appropriate user interfaces, for example.)
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* As a toolkit for experimentation with JPEG and JPEG-like algorithms. Most
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of the individual decisions you might want to mess with are packaged up into
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separate modules. For example, the details of color-space conversion and
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subsampling techniques are each localized in one compressor and one
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decompressor module. You'd probably also want to extend the user interface
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to give you more detailed control over the JPEG compression parameters.
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In particular, we welcome the use of this software as the basis for commercial
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products; no royalty is required.
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ARCHIVE LOCATIONS
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=================
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The "official" archive site for this software is uunet.uu.net (Internet
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address 137.39.1.2 or 192.48.96.2). The most recent released version can
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always be found there in directory graphics/jpeg. This particular version
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will be archived as jpegsrc.v2.tar.Z. If you are on the Internet, you can
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retrieve files from UUNET by anonymous FTP. If you don't have FTP access,
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UUNET's archives are also available via UUCP; contact postmaster@uunet.uu.net
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for information on retrieving files that way.
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Various other Internet sites maintain copies of the UUNET file, which may or
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may not be up-to-date. In Europe, try nic.funet.fi (128.214.6.100; look in
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directory pub/graphics/programs/jpeg).
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You can also obtain this software from CompuServe, in the GRAPHSUPPORT forum
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(GO PICS), library 10; this version will be file jpsrc2.zip.
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If you are not reasonably handy at configuring and installing portable C
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programs, you may have some difficulty installing this package. You may
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prefer to obtain a pre-built executable version. A collection of pre-built
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executables for various machines is currently available for anonymous FTP at
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procyon.cis.ksu.edu (129.130.10.80 --- this number is due to change soon);
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look under /pub/JPEG. The administrators of this system ask that FTP traffic
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be limited to non-prime hours. For more information on this archive, please
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contact Steve Davis (strat@cis.ksu.edu). This collection is not maintained by
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the Independent JPEG Group, and programs in it may not be the latest version.
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SUPPORTING SOFTWARE
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===================
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You will probably want Jef Poskanzer's PBMPLUS image software, which provides
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many useful operations on PPM-format image files. In particular, it can
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convert PPM images to and from a wide range of other formats. You can FTP
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this free software from export.lcs.mit.edu (contrib/pbmplus*.tar.Z) or
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ftp.ee.lbl.gov (pbmplus*.tar.Z). Unfortunately PBMPLUS is not nearly as
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portable as the JPEG software is; you are likely to have difficulty making it
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work on any non-Unix machine.
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If you are using X Windows you might want to use the xv or xloadimage viewers
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to save yourself the trouble of converting PPM to some other format. Both of
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these can be found in the contrib directory at export.lcs.mit.edu.
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There will soon be a new release of xv that incorporates our software and thus
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can read and write JPEG files directly. (NOTE: since xv internally reduces
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all images to 8 bits/pixel, a JPEG file written by xv will not be very high
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quality. Caveat user.)
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SOFTWARE THAT'S NO HELP AT ALL
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==============================
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Handmade Software's shareware PC program GIF2JPG produces files that are
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totally incompatible with our programs. They use a proprietary format that is
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an amalgam of GIF and JPEG representations. However, you can force GIF2JPG
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to produce compatible files with its -j switch, and their decompression
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program JPG2GIF can read our files (at least ones produced with our default
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option settings).
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Unfortunately, most commercial JPEG implementations are also incompatible as
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of this writing, especially programs released before summer 1991. The root of
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the problem is that the ISO JPEG committee failed to specify a concrete file
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format. Many vendors "filled in the blanks" on their own, creating
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proprietary formats that no one else could read. (For example, none of the
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early commercial JPEG implementations for the Macintosh were able to exchange
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compressed files.)
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The file format we have adopted is called JFIF (see REFERENCES). This format
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has been agreed to by a number of major commercial JPEG vendors, and we expect
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that it will become the de facto standard. JFIF is a minimal representation;
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work is also going forward to incorporate JPEG compression into the TIFF
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standard, for use in "high end" applications that need to record a lot of
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additional data about an image. We intend to support JPEG-in-TIFF in the
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future. We hope that these two formats will be sufficient and that other,
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incompatible JPEG file formats will not proliferate.
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Indeed, part of the reason for developing and releasing this free software is
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to help force rapid convergence to de facto standards for JPEG file formats.
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SUPPORT STANDARD, NON-PROPRIETARY FORMATS: demand JFIF or JPEG-in-TIFF!
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REFERENCES
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==========
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The best and most readily available introduction to the JPEG compression
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algorithm is Wallace's article in the April '91 CACM:
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Wallace, Gregory K. "The JPEG Still Picture Compression Standard",
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Communications of the ACM, April 1991 (vol. 34 no. 4), pp. 30-44.
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(Adjacent articles in that issue discuss MPEG motion picture compression,
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applications of JPEG, and related topics.) We highly recommend reading that
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article before trying to understand the innards of any JPEG software.
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For more detail about the JPEG standard you pretty much have to go to the
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draft standard (which is not nearly as intelligible as Wallace's article).
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The standard is not now available electronically; you must order a paper copy
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through ISO. In the US, copies may be ordered from ANSI Sales at (212)
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642-4900. The standard is divided into two parts: Part 1 is the actual
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specification, and Part 2 covers compliance testing methods. The current
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"committee draft" version of Part 1 is titled "Digital Compression and Coding
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of Continuous-tone Still Images, Part 1: Requirements and guidelines" and has
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document number ISO/IEC CD 10918-1. (The alternate number SC2 N2215 should
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also be mentioned when ordering.) This draft is expected to be superseded by
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the Draft International Standard version around the end of November 1991.
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Ordering info will be the same as above, but replace "CD" with "DIS" in the
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document number (alternate number not yet known). The committee draft of
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Part 2 is expected to be available around the end of December 1991. It will
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be titled "Digital Compression and Coding of Continuous-tone Still Images,
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Part 2: Compliance testing" and will have document number ISO/IEC CD 10918-2
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(alternate number not yet known).
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The JPEG standard does not specify all details of an interchangeable file
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format. For the omitted details we follow the "JFIF" conventions, revision
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1.01. A copy of the JFIF spec is available from:
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Literature Department
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C-Cube Microsystems, Inc.
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399A West Trimble Road
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San Jose, CA 95131
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(408) 944-6300
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Requests can also be e-mailed to info@c3.pla.ca.us. The same source can
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supply copies of the draft JPEG-in-TIFF specs.
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If you want to understand this implementation, start by reading the
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"architecture" documentation file. Please read "codingrules" if you want to
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contribute any code.
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LEGAL ISSUES
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============
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The authors make NO WARRANTY or representation, either express or implied,
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with respect to this software, its quality, accuracy, merchantability, or
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fitness for a particular purpose. This software is provided "AS IS", and you,
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its user, assume the entire risk as to its quality and accuracy.
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This software is copyright (C) 1991, Thomas G. Lane.
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All Rights Reserved except as specified below.
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Permission is hereby granted to use, copy, modify, and distribute this
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software (or portions thereof) for any purpose, without fee, subject to these
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conditions:
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(1) If any part of the source code for this software is distributed, then this
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README file must be included, with this copyright and no-warranty notice
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unaltered; and any additions, deletions, or changes to the original files
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must be clearly indicated in accompanying documentation.
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(2) If only executable code is distributed, then the accompanying
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documentation must state that "this software is based in part on the work of
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the Independent JPEG Group".
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(3) Permission for use of this software is granted only if the user accepts
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full responsibility for any undesirable consequences; the authors accept
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NO LIABILITY for damages of any kind.
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Permission is NOT granted for the use of any author's name or author's company
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name in advertising or publicity relating to this software or products derived
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from it. This software may be referred to only as "the Independent JPEG
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Group's software".
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We specifically permit and encourage the use of this software as the basis of
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commercial products, provided that all warranty or liability claims are
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assumed by the product vendor.
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ansi2knr.c is included in this distribution by permission of L. Peter Deutsch,
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sole proprietor of its copyright holder, Aladdin Enterprises of Menlo Park, CA.
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ansi2knr.c is NOT covered by the above copyright and conditions, but instead
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by the usual distribution terms of the Free Software Foundation; principally,
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that you must include source code if you redistribute it. (See the file
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ansi2knr.c for full details.) However, since ansi2knr.c is not needed as part
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of any product generated from the JPEG code, this does not limit you more than
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the foregoing paragraphs do.
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It appears that the arithmetic coding option of the JPEG spec is covered by
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patents owned by IBM and AT&T, as well as a pending Japanese patent of
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Mitsubishi. Hence arithmetic coding cannot legally be used without obtaining
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one or more licenses. For this reason, support for arithmetic coding has been
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removed from the free JPEG software. (Since arithmetic coding provides only a
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marginal gain over the unpatented Huffman mode, it is unlikely that very many
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people will choose to use it. If you do obtain the necessary licenses,
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contact jpeg-info@uunet.uu.net for a copy of our arithmetic coding modules.)
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So far as we are aware, there are no patent restrictions on the remaining
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code.
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We are required to state that
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"The Graphics Interchange Format(c) is the Copyright property of
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CompuServe Incorporated. GIF(sm) is a Service Mark property of
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CompuServe Incorporated."
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TO DO
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=====
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Many of the modules need fleshing out to provide more complete
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implementations, or to provide faster paths for common cases. The greatest
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needs are for (a) decent color quantization, and (b) a memory manager
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implementation that can work in limited memory by swapping "big" images to
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temporary files. I (Tom Lane) am going to work on color quantization next.
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Volunteers to write a PC memory manager, or to work on any other modules, are
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welcome.
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We'd appreciate it if people would compile and check out the code on as wide a
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variety of systems as possible, and report any portability problems
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encountered (with solutions, if possible). Checks of file compatibility with
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other JPEG implementations would also be of interest. Finally, we would
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appreciate code profiles showing where the most time is spent, especially on
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unusual systems.
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Please send bug reports, offers of help, etc. to jpeg-info@uunet.uu.net.
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