GIF

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; later it became a major Web graphic format. GIF's ability to have animation (unlike most still graphic formats) has caused it to gain some Internet notoriety and use in conjunction with "memes".

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 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"). Finally in 2013, the creator of GIF came out publicly in favor of the soft-G pronunciation, but even that didn't end the debate.

Animated GIF
Unlike most other graphic formats, GIF supports multiple-frame animated graphics in addition to single-image graphics. These animated GIFs are often used on web pages, sometimes for good reasons, sometimes as a poor man's video format, and all too often (especially during the early wave of Web popularity) to animate clip art and other images that would be better left motionless. In the 2010s, the use of animated GIFs for spreading Internet memes had a sudden and intense burst of popularity, and could sometimes be used in a very artistic manner.

The GIF features used for animation seem to have been intended for slideshows, and not for the type of animation they came to be used for. In fact, the specification clearly states that "[GIF] is not intended as a platform for animation." The only missing element – a loop count – was supplied by Netscape's looping extension (see below).

Version 87a
The original version, released 1987-06-15. As a reference point, it supports:
 * Multiple images in a single file
 * Interlaced images
 * Background colors
 * Extensions, but each extension would have to be approved by CompuServe in order to avoid conflicts

Version 89a
Released 1990-07-31. Adds support for:
 * Binary transparency
 * Animation, though there is no way to indicate whether the animation should loop
 * Application extensions that don't require central coordination
 * Comment extensions
 * Plain Text extensions (an obsolete feature)

Other versions
There are no other official GIF version numbers.

Jeff's Image Format is a GIF-like format identified by JIF99a. The file command's database suggests that GIF-like formats identified by GIF94z ("ZIF") and FGF95a ("FGF") were experimented with.

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. This technique is inefficient, 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


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 ).



Same as "NETSCAPE2.0" looping extension.



Stores an ICC profile.



Stores XMP metadata.



Used by the DOS program Fractint to save additional data (fractal parameters). The only documentation about this may be the source code. See also FRA.

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

Specifications

 * GIF87a
 * Text (W3C)
 * Text (Stanford)
 * Text (textfiles.com)
 * GIF89a
 * Text (W3C)
 * Text (textfiles.com)
 * HTML (rug.nl)
 * Netscape Looping Application Extension
 * GIF Application Extension: NETSCAPE2.0 (alternate link from archive.org)

Software
Support for GIF is ubiquitous. The software listed here has been arbitrarily selected.
 * GIFLIB
 * Netpbm: ppmtogif, pamtogif, giftopnm
 * Animated GIF converter/player for Apple II series (Video of it in action)