The optional interlacing feature, which stores image scan lines out of order in such a fashion that even a partially downloaded image was somewhat recognizable, also helped GIF's popularity, as a user could abort the download if it was not what was required. The feature of storing multiple images in one file, accompanied by control data, is used extensively on the Web to produce simple animations. In 2004, all patents relating to the proprietary compression used for GIF expired. Controversy over the licensing agreement between Unisys and CompuServe in 1994 spurred the development of the Portable Network Graphics (PNG) standard. While GIF was developed by CompuServe, it used the Lempel–Ziv–Welch (LZW) lossless data compression algorithm patented by Unisys in 1985. In September 1995 Netscape Navigator 2.0 added the ability for animated GIFs to loop. GIF was one of the first two image formats commonly used on Web sites, the other being the black-and-white XBM. By December 1987, for example, an Apple IIGS user could view pictures created on an Atari ST or Commodore 64. The two versions can be distinguished by looking at the first six bytes of the file (the " magic number" or signature), which, when interpreted as ASCII, read "GIF87a" or "GIF89a", respectively.ĬompuServe encouraged the adoption of GIF by providing downloadable conversion utilities for many computers. As there is little control over display fonts, however, this feature is rarely used.
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