Casio V-N500 - Manual (Page 230)

E-228
It appears that the arithmetic coding option of the JPEG spec is covered by patents owned
by IBM, 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 implementations will
support it.) So far as we are aware, there are no patent restrictions on the remaining code.
The IJG distribution formerly included code to read and write GIF fi les.
To avoid entanglement with the Unisys LZW patent, GIF reading support has been
removed altogether, and the GIF writer has been simplifi ed to produce “uncompressed
GIFs”. This technique does not use the LZW algorithm; the resulting GIF fi les are larger
than usual, but are readable by all standard GIF decoders.
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.”
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 fi le
containing a revised version of Wallace’s article is available at ftp://ftp.uu.net/graphics/
jpeg/wallace.ps.gz. The fi le (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 and Jean-loup Gailly, published by M&T Books
(New York), 2nd ed. 1996, ISBN 1-55851-434-1. 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...