Graphics Interchange Format

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Graphics Interchange Format
A rotating globe in GIF format. The gradient blue areas of this image transition choppily, a common artifact produced when dithering is not employed.
File extension:	.gif
MIME type:	image/gif
Type code:	GIF
Uniform Type Identifier:	com.compuserve.gif
Magic:	GIF87a/GIF89a
Developed by:	CompuServe
Type of format:	Raster graphics image format

An example of a GIF image. The dithering process used to overcome the format's 256-color limitation makes the image appear coarse-grained.

The Graphics Interchange Format (GIF) is an 8-bit-per-pixel bitmap image format that was introduced by CompuServe in 1987 and has since come into widespread usage on the World Wide Web due to its wide support and portability.

The format uses a palette of up to 256 distinct colors from the 24-bit RGB color space. It also supports animations and allows a separate palette of 256 colors for each frame. The color limitation makes the GIF format unsuitable for reproducing color photographs and other images with continuous color, but it is well-suited for more simple images such as graphics or logos with solid areas of color.

GIF images are compressed using the Lempel-Ziv-Welch (LZW) lossless data compression technique to reduce the file size without degrading the visual quality. This compression technique was patented in 1985. Though the relevant patents have all since expired, the controversy over the licensing agreement between the patent holder, Unisys, and CompuServe in 1994 led to the development of the Portable Network Graphics (PNG) standard.

[edit] History

CompuServe introduced the GIF format in 1987 to provide a color image format for their file downloading areas, replacing their earlier RLE format, which was black and white only. GIF became popular because it used LZW data compression, which was more efficient than the run-length encoding that formats such as PCX and MacPaint used, and fairly large images could therefore be downloaded in a reasonable amount of time, even with very slow modems.

The original version of the GIF format was called 87a. In 1989, CompuServe devised an enhanced version, called 89a,^[1] that added support for multiple images in a stream, interlacing and storage of application-specific metadata. The two versions can be distinguished by looking at the first six bytes of the file, which, when interpreted as ASCII, read "GIF87a" and "GIF89a", respectively.

GIF was one of the first two image formats commonly used on Web sites, the other being the black and white XBM.^{[citation needed]} JPEG came later with the Mosaic browser.

The GIF89a feature of storing multiple images in one file, accompanied by control data, is used extensively on the web to produce simple animations. The optional interlacing feature, which stored image scan lines out of order in such a fashion that even a partially downloaded image was somewhat recognizable, also helped GIF's popularity,^{[citation needed]} as a user could abort the download if it was not what was required.

[edit] Pronunciation

Most people pronounce GIF with a hard "G" reflecting the way it is pronounced in its own acronym (Graphics Interchange Format) ^[2]. However, the creators of the format pronounced GIF with a soft "g", IPA: [dʒ], as in "George". According to the creator of the GIF format, Steve Wilhite, the pronunciation deliberately echoes that of an American peanut butter brand, Jif, and the employees of CompuServe would often say "Choosy developers choose GIF", spoofing this brand's television commercials.^{[citation needed]} This pronunciation was also identified by CompuServe in their documentation of a graphics display program called CompuShow.^[3]. Both pronunciations are given as correct by the Oxford English Dictionary^[4] and the American Heritage Dictionary.^[5]

[edit] Usage

An example of a short GIF animation made with a digital camera

• GIFs are suitable for sharp-edged line art (such as logos) with a limited number of colors. This takes advantage of the format's lossless compression which preserves very sharp edges (in contrast to JPEG).
• GIFs can also be used to store low-color sprite data for games.
• GIFs are used for small animations and low-resolution film clips.
• In view of the limitation on the GIF formation to 256 colors, JPEG is a more commonly used format for digital photographs. JPEGs can save information on more than 16 million different colors and use more aggressive lossy compression which has a less noticeable effect on photographs than it does on images with sharp edges.
• In circumstances where speed is more important than reduced file size, uncompressed bitmap formats such as Windows bitmap are more commonly used than the GIF format, since uncompressed bitmaps contain raw pixel information and can be displayed very quickly.
• The PNG format is becoming an increasingly popular replacement for GIF images since it uses better compression techniques and does not have a limit of 256 colors, but PNGs do not support animations. The MNG and APNG formats, both derived from PNG, support animations but aren't widely used.

[edit] Palettes

GIF is palette based: although any palette selection can be one of millions of shades, the maximum number that can be used in a frame is 256. These are stored in a "palette", a table that associates each palette selection number with a specific RGB value. The limitation to 256 colors seemed reasonable at the time of GIF's creation because few people could afford the hardware to display more. Simple graphics, line drawings, cartoons, and grey-scale photographs typically need fewer than 256 colors. In addition, one of the colors in the palette can optionally be set as fully transparent.

There exist ways to dither or diffuse photographs by using pixels of 2 or more different colors to approximate an in-between color, but this transformation inevitably loses some detail. The algorithms used to select the palette and to perform the dithering vary widely in output quality. Additionally, dithering significantly reduces the image's compressibility and thus works contrary to GIF's main purpose.

In the early days of graphical web browsers, graphics cards with 8-bit buffers (allowing only 256 colors) were common and it was fairly common to make GIF images using the websafe palette which was based on the common subset of the standard Windows and Macintosh palettes.^{[citation needed]} This ensured predictable display but severely limited the choice of colors. Now that 24-bit graphics cards are the norm, optimized palettes make less sense when creating images, though some web designers still advise the use of the web safe palette.^{[citation needed]}

[edit] True color

Although the standard GIF format is limited to 256 colors, there is a hack^[6] that can overcome this limitation under certain circumstances.

GIF89a was designed based on the principle of rendering images (known as frames when used for animation) to a logical screen. Each image could optionally have its own palette, and the format provides flags to specify delay and waiting for user input between them (the latter is not widely supported by viewers). This is the feature that is used to create animated GIFs, but it can also be used to store a 24-bit RGB (truecolor) image by splitting it up into pieces small enough to be encoded into a 256 color palette and setting up the GIF to render these with no delay on the logical screen.^[7][8] However, most web browsers seem to assume that this multi-image feature will only be used for animation and insert a minimum delay between images. There will also be some file size bloat from doing this. There are few tools around that can easily produce 24-bit GIFs (e.g. ANGIF or SView5) - however it is rarely an appropriate format unless there is absolutely no other option.

This short section requires expansion.

[edit] Unisys and LZW patent enforcement

In 1977 and 1978, Jacob Ziv and Abraham Lempel published a pair of papers on a new class of lossless data-compression algorithms, now collectively referred to as "LZ77" and "LZ78". In 1983, Terry Welch developed a fast variant of LZ78 which was named LZW.^[9][10]

Welch filed a patent application for the LZW method in June 1983. The resulting patent, US patent 4558302, granted in December 1985, was assigned to Sperry Corporation who subsequently merged with Burroughs Corporation in 1986 and formed Unisys.^[9] Further patents were obtained in the United Kingdom, France, Germany, Italy, Japan and Canada.

In June 1984, an article by Welch was published in the IEEE magazine which publicly described the LZW technique for the first time.^[11] LZW became a popular data compression technique and, when the patent was granted, Unisys entered into licensing agreements with over a hundred companies.^[9][12]

The popularity of LZW led CompuServe to choose it as the compression technique for their GIF format, developed in 1987. At the time, CompuServe were not aware of the patent.^[9] Unisys became aware that the GIF format used the LZW compression technique and entered into licensing negotiations with CompuServe in January 1993. The subsequent agreement was announced on December 24, 1994.^[10] Unisys stated that they expected all major commercial on-line information services companies employing the LZW patent to license the technology from Unisys at a reasonable rate, but that they would not require licensing, or fees to be paid, for non-commercial, non-profit GIF-based applications, including those for use on the on-line services.^[12]

Following this announcement, there was widespread condemnation of CompuServe and Unisys, and many software developers threatened to stop using the GIF format. The PNG format was developed in 1995 as an intended replacement.^[9][10][11] However, obtaining support from the makers of Web browsers and other software for the PNG format proved difficult and it was not possible to replace the GIF format entirely, although PNG has gradually increased in popularity.^[9]

In August 1999, Unisys changed the details of their licensing practice, announcing the option for owners of Billboard and Intra net Web sites to obtain licenses on payment of a one-time license fee of $5000 or $7500.^[13] Such licenses were not required for website owners or other GIF users who had used licensed software to generate GIFs. Nevertheless, Unisys was the subject of thousands of online attacks and abusive emails from users believing that they were going to be charged $5000 or sued for using GIFs on their websites.^[14]. Despite giving free licenses to hundreds of non-profit organizations, schools and governments, Unisys was completely unable to generate any good publicity and continued to be vilified by individuals and organizations such as the League for Programming Freedom who started the "Burn All GIFs" campaign^[15]

The US LZW patent expired on June 20, 2003. The counterpart patents in the United Kingdom, France, Germany and Italy expired on June 18, 2004, the Japanese counterpart patents expired on June 20, 2004 and the counterpart Canadian patent expired on July 7, 2004.^[16] Consequently, while Unisys has further patents and patent applications relating to improvements to the LZW technique,^[16] the GIF format may now be used freely.

[edit] Alternatives

Portable Network Graphics (PNG) was designed as a replacement for the GIF format in order to avoid infringement of Unisys' patent on the LZW compression technique.^[9] PNG offers better compression and more features than GIF. The format is more suitable than GIF in instances where true-color imaging, alpha transparency, or a lossless data format are required. MNG was originally developed as a PNG-based solution for animations, but has not been widely adopted. The GIF format is still preferred for animation over PNG, although true Animated Portable Network Graphics are under development by Mozilla and will be supported in the upcoming Firefox 3 release.^[17][18]

Although it took time for the PNG format to be supported, new web browsers support the PNG format and GIF images can usually be replaced by PNG images if desired. However, Internet Explorer versions 6 and earlier do not support PNG's alpha channel transparency feature without using Microsoft-specific HTML extensions.^[19] Using standard HTML <img> tags for PNG images in Internet Explorer can produce a look different from that intended. Internet Explorer 7 supports alpha channel transparency without needing extensions.^[20]

There have been claims that PNG files are generally larger than GIFs.^{[citation needed]} PNG files can indeed be much larger than GIF files in situations where a GIF and a PNG file were created from a high-quality master image, as PNG is capable of storing more color depth and transparency information than GIF. However, identical 8-bit (or lower) image data rendered in PNG and GIF formats should yield similar sizes. Misinformation about PNG efficiency can generally be traced back to poor PNG support in older versions of some image manipulation programs, (for example Adobe Photoshop did not optimize PNGs for reduced color palettes by default).^{[citation needed]}MNG, a variant of PNG that supports animation, reached version 1.0 in 2001, but few applications support it. Animated GIF remains widely used as many applications are capable of creating the files, and it remains the only animated image format capable of being rendered in nearly all modern web browsers without the use of a plug-in.^{[citation needed]} Nevertheless, embedded Flash objects, MPEGs and other video formats are used in place of animated GIFs in many websites. Other approaches, such as individual frames served by AJAX, or SVG images may be animated via JavaScript. In 2004, a proposed extension to the PNG format called APNG was suggested. It was to provide the ability to animate PNG files, while retaining backwards compatibility in decoders that cannot understand the animation chunk. Older decoders would simply render the first frame of the animation.

[edit] See also

• Comparison of graphics file formats
• Comparison of layout engines (graphics)
• Image file formats
• PNG
• JPEG
• Software patent

[edit] References

[edit] External links

• GIF89a specification
• The GIF Pronunciation Page
• GIF Animation on the WWW - technical explanation of the GIF89a format and how it allows animation
• LZW GIF decoding by Arturo San Emeterio Campos
• LZW and GIF explained
• Replacing a Dictionary with a Square Root - suggestions for compressing GIFs without using LZW compression
• Web Graphics discusses and illustrates the differences between GIF, JPEG and PNG formats