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LZW
Last modified: Friday, February 11, 2000
Short for Lempel-Zif-Welch, a popular data compression technique developed in 1977 by J. Ziv and A Lempel, and later refined by Terry Welch. It is the compression algorithm used in the GIFgraphics file format, which is one of the standard graphic formats used by CompuServe and the World Wide Web.
The patent for LZW is owned by Unisys, which for many years allowed anyone to use the algorithm for free. Then in 1995, Unisys suddenly decided to charge a license fee. There was an uproar from the Compuserve and Web communities, and Unisys backed down somewhat, though they still enforce the licensing requirement for commercial applications.
The GIF Controversy: A Software Developer's Perspective Article on the GIF controversy seen from a software developer's perspective. Includes a collection of information from the history of the controversy to the most recent events.
League for programming freedom Provides news, position statements and legal resouces covering software patent issues and user interface copyrights. Includes information on the LZW patent.
LZW Patent FAQs Answers various questions about the LZW patent, including the GIF specification.
The LZW Algorithm Detailed description of encoding and decoding algorithms as well as practical characteristics.
Unisys LZW page Provides license information on GIF and other LZW-based technologies
