Editing Lempel–Ziv–Welch (section)

== Patents ==
{{main|Graphics Interchange Format#Unisys and LZW patent enforcement}}
Various [[patent]]s have been issued in the [[United States]] and other countries for LZW and similar algorithms. LZ78 was covered by {{US patent|4464650}} by Lempel, Ziv, Cohn, and Eastman, assigned to [[Sperry Corporation]], later [[Unisys]] Corporation, filed on August 10, 1981. Two US patents were issued for the LZW algorithm: {{US patent|4814746}} by [[Victor S. Miller]] and [[Mark N. Wegman]] and assigned to [[IBM]], originally filed on June 1, 1983, and {{US patent|4558302}} by Welch, assigned to Sperry Corporation, later Unisys Corporation, filed on June 20, 1983.

In addition to the above patents, Welch's 1983 patent also includes citations to several other patents that influenced it, including two 1980 Japanese patents ([https://patents.google.com/patent/JPS5719857A/en JP9343880A] and [https://patents.google.com/patent/JPS57101937A/en JP17790880A]) from [[NEC]]'s Jun Kanatsu, {{US patent|4021782}} (1974) from John S. Hoerning, {{US patent|4366551}} (1977) from Klaus E. Holtz, and a 1981 German patent ([https://patents.google.com/patent/DE3118676C2/en DE19813118676]) from Karl Eckhart Heinz.<ref>{{US patent|4558302}}</ref>

In 1993–94, and again in 1999, Unisys Corporation received widespread condemnation when it attempted to enforce licensing fees for LZW in GIF images.  The 1993–1994 Unisys-CompuServe controversy ([[CompuServe]] being the creator of the GIF format) prompted a [[Usenet]] comp.graphics discussion ''Thoughts on a GIF-replacement file format'', which in turn fostered an email exchange that eventually culminated in the creation of the patent-unencumbered [[Portable Network Graphics]] (PNG) file format in 1995.

Unisys's US patent on the LZW algorithm expired on June 20, 2003,<ref name=LZWPatentInfo>{{cite web |title=LZW Patent Information |url=http://www.unisys.com/about__unisys/lzw/ |work=About Unisys |publisher=Unisys |access-date=March 6, 2014 |archive-url=https://web.archive.org/web/20090626052026/http://www.unisys.com/about__unisys/lzw/ |archive-date=June 26, 2009}}</ref> 20 years after it had been filed. Patents that had been filed in the United Kingdom, France, Germany, Italy, Japan and Canada all expired in 2004,<ref name=LZWPatentInfo/> likewise 20 years after they had been filed.