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LZW Compression Encoding

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Format Description Properties Explanation of format description terms

Identification and description Explanation of format description terms

Full name LZW (Lempel-Ziv-Welch) Image Compression Encoding
Description A lossless compression algorithm for digital data of many kinds, named for the creators Abraham Lempel and Jacob Ziv, and a later contributor, Terry Welch. LZW is based on a translation table that maps strings of input characters into codes. Through its incorporation in the graphics file formats GIF_89a and TIFF_LZW, LZW has come to be strongly associated with image compression.
Production phase Used for initial-, middle- and final-state (end-user delivery) purposes.
Relationship to other formats
    Used by TIFF_LZW, TIFF with LZW compression
    Used by GIF_89a, Graphics Interchange Format, Version 89a
    Used by Other file or wrapper formats, not documented at this time

Local use Explanation of format description terms

LC experience or existing holdings None (or very little)
LC preference See the Library of Congress Recommended Formats Statement for format preferences for still image works.

Sustainability factors Explanation of format description terms


Proprietary standard ("open"). Predecessor algorithms were developed by Abraham Lempel and Jacob Ziv; LZW is the refined version finalized by Terry Welch, an employee of Sperry Corporation (later merged with Burroughs to form the Unisys Corporation).

    Documentation The definitive descriptive article by Welch: "A Technique for High-Performance Data Compression." Patent 4,558,302 was granted in 1985; key in number at Patent Number Search at the U.S. Patent and Trademark Office.

Widely adopted. Many software tools exist for encoding and decoding. Generally not natively supported by Web browsers.

    Licensing and patents

LZW is notorious because of late-in-the-game patent-protection actions taken by Unisys beginning in late 1994, perceived as having an adverse affect on the Web due to that medium's widespread use of GIF images, which employ LZW compression. Unisys's US patent expired in June 2003, and its European and Japanese patents expired in June 2004. In 2007, the company's LZW Patent and Software Information Web page stated that the "Unisys Corporation holds and has patents pending on a number of improvements on the inventions claimed in the above-expired patents." As of January 2012, the Unisys Web site has no mention of LZW.


Relatively transparent but depends upon algorithms and tools to read.

Self-documentation Not applicable.
External dependencies None.
Technical protection considerations Not applicable.

Quality and functionality factors Explanation of format description terms

Still Image
Normal rendering Good support.
Clarity (high image resolution) Excellent, given that LZW offers lossless compression.
Color maintenance Not applicable.
Support for vector graphics, including graphic effects and typography Not applicable.
Functionality beyond normal rendering None.

File type signifiers and format identifiers Explanation of format description terms

Tag Value Note
Filename extension Not applicable.   
Internet Media Type Not applicable.   
Magic numbers Not applicable.   
Pronom PUID x-cmp/12
Wikidata Title ID Q2681

Notes Explanation of format description terms

History The ironically titled "Sad day . . . GIF patent dead at 20" includes a useful chronology compiled by an individual unhappy with Unisys's patent protections.

Format specifications Explanation of format description terms

Useful references


Books, articles, etc.

Last Updated: 03/23/2023