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Section - [18] What about arithmetic coding?
The JPEG spec defines two different "back end" modules for the final output of compressed data: either Huffman
coding or arithmetic coding is allowed. The choice has no impact on image quality, but arithmetic coding usually
produces a smaller compressed file. On typical images, arithmetic coding produces a file 5 to 10 percent smaller
than Huffman coding. (All the file-size numbers previously cited are for Huffman coding.)
Unfortunately, the particular variant of arithmetic coding specified by the JPEG standard is subject to patents owned
by IBM, AT&T, and Mitsubishi. Thus *you cannot legally use JPEG arithmetic coding* unless you obtain licenses
from these companies. (Patent law's "experimental use" exception allows people to test a patented method in the
context of scientific research, but any commercial or routine personal use is infringement.)
I recommend that people not use JPEG arithmetic coding; the space savings isn't great enough to justify the potential
legal hassles. In particular, arithmetic coding *should not* be used for any images to be exchanged on the Internet.
Even if you don't care about US patent law, other folks do. |
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