The GIF image format is widely used on the World Wide Web. To make a GIF image, the LZW algorithm is required. Until 2004, this algorithm had problems with patents that were aggressively enforced.
As of 2004, that patent is expired and GIF is generally considered to be free of patent problems.
 Related pages on en.swpat.org
- Burn All Gifs - an old LPF project encouraging people not to use .gif images
- Wikipedia: GIF - Unisys_and_LZW_patent_enforcement
- James S. Huggins' summary
- The story of PostScript, GIF, and PNG, told by Richard Stallman
- Wikipedia: Submarine patents and Patent ambush
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