LZW CompressionLZW is a method of compression named after Abraham Lempel, Jakob Ziv and Terry Welch, the scientists who developed this compressionalgorithm.
In vastly simplified terms the way that LZW works is to build a dictionary of repeating terms, and to express these repeating terms as references to that dictionary. For example, in the sentence "the quick brown fox jumped over the lazy dog", the word "the" is a repeating pattern that might be encoded in this way.
LZW is very effective (and fast) for data which contains lots of repeating patterns, for example images with solid color areas. However, LZW is really unsuited for images which contain a lot of noise, and in these cases may even increase file size.
LZW is, or can be used in a number of file formats including GIF and TIFF.
LZW is (or was) patented by Unisys. This resulted in a certain amount of controversy in the context of GIF files, when GIF files are compressed using an LZW compression scheme, which attracted a large amount of controversy when Unisys announced this algorithm was patented, and charged license fees for software developers using this algorithm. However, according to The GIF Controversy: A Software Developer's Perspective, these patents have now expired as of 20 June 2004
A technical discussion of the LZW algorithm can be found in Chapters 1 and 7 of Graphics File Formats† by David C. Kay and John R. Levine.
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