GIF
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== Format == | == Format == | ||
| − | A GIF file has | + | A GIF file has a header (consisting of a signature, a ''screen descriptor'', and optionally a ''global color table''), followed by a sequence of tagged blocks of various types. |
Block type 0x21 is an ''extension''. Each extension has a byte indicating its type. Extension type 0xff is an ''application extension'', which can be used to store arbitrary data. | Block type 0x21 is an ''extension''. Each extension has a byte indicating its type. Extension type 0xff is an ''application extension'', which can be used to store arbitrary data. | ||
Revision as of 20:25, 4 May 2013
Graphics Interchange Format (GIF) was introduced by the CompuServe online service in 1987, intended to provide a consistent and compact format for graphics to be downloaded on that service. Since the specifications were openly released, the format gained wide use in graphics software and on online services and bulletin board systems (BBSs), not just CompuServe.
In 1994, it became widely known that the format was encumbered by the use of the patent-protected LZW compression technology, for which its owner, Unisys, was demanding licencing terms and royalties for certain sorts of uses. This made GIF a less-than-free format, spurring a desire on the part of some people for an unencumbered format, which led to the creation of the PNG format. However, it took several years for PNG to get widespread support in software, and in the meantime the World Wide Web experienced meteoric growth with GIF still used as the primary graphics format (alongside JPEG), though eventually PNG did become widespread on the Web as well. The patent in question expired in the US in 2003, and in other countries in 2004, so it is no longer an issue.
Unlike most other graphic formats, GIF supports multiple-frame animated graphics in addition to single-image graphics. These animated GIFs often turn up on the Web, though they are usually thought of as somewhat "retro" these days because they remind people of the excesses of the early wave of Web popularity when such tricks were common on pages on services such as GeoCities.
People argue a lot over whether to pronounce "GIF" with a hard or a soft G. Both pronunciations are acceptable to the Oxford American Dictionaries, which named it the word of the year in 2012, in its supposed usage as a verb (which they don't seem to actually use in a sentence anywhere in their announcement). Pedants say that the hard "g" is proper due to it standing for "graphics", which has a hard "g", but others cite the normal English pattern of pronouncing "g" soft when followed by an "i" (though, like most English spelling and pronunciation rules, it has exceptions like "gift"). The peanut-butter brand Jif, with its slogan "Choosy moms choose Jif", may also have some influence in the soft-pronunciation direction (though it is actually spelled with a "J").
Contents |
Format
A GIF file has a header (consisting of a signature, a screen descriptor, and optionally a global color table), followed by a sequence of tagged blocks of various types.
Block type 0x21 is an extension. Each extension has a byte indicating its type. Extension type 0xff is an application extension, which can be used to store arbitrary data.
An application extension's specific type is then indicated by an identifier of exactly 8 ASCII characters, plus a three-byte "authentication code" to reduce the chance of a conflict. In effect, this means it has an 11-byte identifier.
Known application extensions
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"NETSCAPE" 0x32 0x2e 0x30 ("NETSCAPE2.0")
Used in animated GIFs. Indicates the number of times to repeat the animation.
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"ICCRGBG1" 0x30 0x31 0x32 ("ICCRGBG1012")
Stores an ICC profile.
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"XMP Data" 0x58 0x4d 0x50 ("XMP DataXMP")
Stores XMP metadata.
-
"fractint" ?? ?? ??
Used by the DOS program FractInt to save additional data (fractal parameters). The only documentation about this may be the source code.
Specifications
- GIF87a (1987-06-15)
- GIF89a (1990-07-31)
- GIF Application Extension: NETSCAPE2.0
