GIF

Revision as of 18:07, 5 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.

Patents

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.

Pronunciation

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^[1] 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").

Animated GIF

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.

Color format

GIF images are always paletted. The number of colors in a palette can be any power of 2 from 2 to 256.

If a GIF file contains multiple images, each may have its own palette. This makes it possible for animated GIFs to construct frames that exceed the usual limit of 256 colors^[2]. This technique is inefficient, though, and may result in very large files.

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. GIF version 89a defines extension type 0xff to be an application extension, which can be used to store arbitrary data.

An application extension's specific type is given by an application identifier consisting 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

• "NETSCAPE" 0x32 0x2e 0x30 ("NETSCAPE2.0")

If the first byte of application data has value 0x01, this is a looping extension used in animated GIFs. It indicates the number of times to repeat the animation.

If the first byte is not 0x01, the extension is probably of no interest, but apparently at least one such extension was defined (0x02 = Netscape Buffering Extension^[3]).

• "ANIMEXTS" 0x31 0x2e 0x30 ("ANIMEXTS1.0")

Same as "NETSCAPE2.0" looping extension.

• "ICCRGBG1" 0x30 0x31 0x32 ("ICCRGBG1012")

Stores an ICC profile.

• "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^[4]. See also FRA (Fractint).

Identification

GIF files begin with ASCII characters "GIF87a" (version 87a), or "GIF89a" (version 89a).

Specifications

• GIF87a (1987-06-15)
  • Text (W3C)
  • Text (Stanford)
• GIF89a (1990-07-31)
  • Text
  • HTML
• Netscape Looping Application Extension
• GIF Application Extension: NETSCAPE2.0 (alternate link from archive.org)

References

1. ↑ GIF (as a verb) is dictionary’s word of the year
2. ↑ True-Color GIF Example
3. ↑ Netscape Buffering Application Extension
4. ↑ Fractint source code: encoder.c

External links

• Wikipedia:Graphics Interchange Format
• What's in a GIF
• Gifology: Understanding GIF Files & GIF Animation
• All About GIF89a
• Archive format info, including GIF
• GIF is Oxford Dictionary Word Of 2012