Graphics Interchange Format
There are at least two rarely-used methods[6][7] that can generate a GIF that, if decoded according to the GIF89a standard, will produce an animation that ends with a 24-bit RGB truecolor image.
GIF89a was designed based on the principle of rendering images (known as frames when used for animation) to a logical, fixed-size screen. Each image could optionally have no delay after it is rendered, and could have its own 256-color palette. Also, each image need not fill the entire logical screen, and the animation can cease after the last frame; it need not begin again. The multi-frame, zero-delay, and unique-palette features, optionally combined with transparency, allow for each image to replace only a portion of the previous image's pixel data. When used without looping, a more-than-256-color final result can be achieved.
For example, a GIF can be encoded to render as a series of overlapping full-screen images, each image filling in color that wasn't in the previous one. Transparent pixels can be used to preserve colors from previous images.[8]
A similar method that doesn't use transparency is to encode the GIF to render as a series of less-than-full-screen images adjacent to each other, rather than overlapping.[9]
These methods are not widely supported by GIF-generating software, and Web browsers and other image viewers may not contain completely compliant GIF89a implementations, so their ability to display such GIFs accurately may be limited.
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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.[10][11]
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.[10] 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.[12] LZW became a popular data compression technique and, when the patent was granted, Unisys entered into licensing agreements with over a hundred companies.[10][13]
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.[10] 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.[11] 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.[13]
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.[10][11][12] 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.[10]
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.[14] 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.[15]. 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[16]
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.[17] Consequently, while Unisys has further patents and patent applications relating to improvements to the LZW technique,[17] the GIF format may now be used freely.
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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.[10] 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.[18][19]
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.[20] 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. No versions of Internet Explorer, however, support the gamma feature of PNG images, and the display of these images may be of the wrong tint.[21]
PNG image files are generally smaller than GIF files of the same image quality, due to the more efficient compression techniques used in PNG encoding. 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, for identical 8-bit (or lower) image data, PNG-format image files are almost always smaller than the equivalent GIF. 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.[22] 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.
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See also
- Comparison of graphics file formats
- Comparison of layout engines (graphics)
- Image file formats
- Software patent
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References
- ^ http://www.w3.org/Graphics/GIF/spec-gif89a.txt
- ^ Say-So: Gif of Jif?
- ^ The FAQ section in the documentation for version 8.33 of CompuShow states: The GIF (Graphics Interchange Format), pronounced "JIF", was designed by CompuServe and the official specification released in June of 1987.[1]
- ^ Oxford English Dictionary. Oxford University Press. Retrieved on 2007-04-15.}
- ^ American Heritage Dictionary. Houghton-Mifflin. Retrieved on 2007-04-15.
- ^ GIF 24 Bit (truecolor) extensions
- ^ Itsagif Layers
- ^ This method is demonstrated by Pedagoguery Software's Itsagif software for Windows and older Macintosh OSes.
- ^ This method is explained in detail by Andreas Kleinert in his GIF 24 Bit (truecolor) extensions document, and is demonstrated on Philip Howard's True-Color GIF Example Web page, which uses his angif C library. The demo uses blocks of size 16 by 16 pixels, allowing for up to 256 unique colors, 1 per pixel, the maximum each block's palette can support.
- ^ a b c d e f g History of the Portable Network Graphics (PNG) Format by Greg Roelofs
- ^ a b c Sad day... GIF patent dead at 20
- ^ a b The GIF Controversy: A Software Developer's Perspective
- ^ a b Unisys Clarifies Policy Regarding Patent Use in On-Line Service Offerings - archived by League for Programming Freedom
- ^ LZW Software and Patent Information - clarification of 2 September 1999
- ^ Unisys Not Suing (most) Webmasters for Using GIFs - Slashdot investigation into the controversy
- ^ Burn All GIFs - A project of the League for Programming Freedom
- ^ a b License Information on GIF and Other LZW-based Technologies
- ^ APNG Specification - MozillaWiki
- ^ Mozilla Labs » Blog Archive » Better animations in Firefox 3
- ^ AlphaImageLoader Filter
- ^ http://msdn.microsoft.com/workshop/essentials/whatsnew/whatsnew_70_sdk.asp What's New in Internet Explorer 7
- ^ Adobe Photoshop CS2 for the Web (book)
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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
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