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LZW(Lempel-Ziv-Welch) A data compression method that stems from techniques introduced by Jacob Ziv and Abraham Lempel. LZW has been used in many hardware and software products, including V.42bis modems, GIF, TIF and PDF files and PostScript Level 2. The LZ77 method creates pointers back to repeating data, while LZ78 creates a dictionary of repeating phrases with pointers to those phrases.
The Welch in Lempel-Ziv-Welch is for Terry Welch, the Unisys researcher who created an enhanced version of these methods that was patented by Unisys. Unisys collected royalties from thousands of software publishers whose applications used this algorithm until the patents expired in 2004. This was the most widely licensed data compression patent in history. See PNG.