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Foil 13 LZW (Lempel-Ziv-Welch) Compression

From Image Format Basics CPS606fall96 -- Fall Semester 96. by Nancy J. McCracken * See also color IMAGE

LZW is the most commonly used dictionary scheme, and is patented by Welch at Unisys.
It is adaptive and constructs the table while it is compressing (unlike the Huffman schemes, which compute the code table on one scan and compress on the next).
An LZW compressor starts with a dictionary that has one entry for each single pixel value. For example, if the pixels are 8 bits, the original dictionary has 256 entries. Each time it sees a new sequence of inputs, it adds a new entry to the dictionary. If it sees a pattern that has been seen before, it uses a previous entry.



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