The Graphics Interchange Format
The Graphics Interchange Format (GIF) is an image format introduced in 1987 by CompuServe and has since come into extensive usage on the World Wide Web due to its portability and comprehensive support.
The form gives provisions of up to 8 bits per pixel for each image, permitting a single image to reference its palette of up to 256 different colors chosen from the 24-bit RGB color space. It also backs animations and provides a distinct palette of up to 256 colors for every frame. These palette limits make the GIF format less appropriate for replicating color photographs and other images with incessant color. But it is compatible with easier images like logos or graphics with solid fields of color. GIF images are condensed using the Lempel-Ziv-Welch (LZW) which is a lossless data compression technique that reduces the file size without tainting the visual quality.
CompuServe came up with the GIF format in 1987 to present a color image format for their file downloading areas, substituting their former run-length encoding (RLE) format. The RLE was white and black only. GIF became famous as it used LZW data compression, which was more efficient than the run-length encoding. This also affected downloading of relatively large images in sensibly short time, even with gradual modems.
The initial version of the GIF format was referred to as 87a. In 1989, CompuServe revealed an amended version, called 89a which supported for animation setbacks, background with transparent colors, and storage of application-particular metadata. Additionally, the 89a description allows for incorporating of text labels as text, but as there is little regulation over display fonts, this characteristic is not broadly used.
The characteristic of storing various images in one file, supplemented by control data, is used expansively on the Web to create simple animations. The optional interlacing feature, which stocks image scan lines ruined in such a fashion that even a partly downloaded image was somewhat noticeable, also aided GIF's fame, as a user could stop the download if it were not what was needed.
The Word GIF is found in the fresher editions of various dictionaries as a noun. The American annex of the Oxford University Press in 2012 recognized GIF as a verb as well. This meant that to create a GIF file as in "GIFing was the perfect medium for sharing scenes from the Summer Olympics. The press's lexicographers selected it their word of the year.
The inventors of the format verbalized GIF as "Jif" with a soft "G" /ˈdʒɪf/ as in "gin". Steve Wilhite declares that the envisioned articulation intentionally resonates the American peanut butter brand, Jif, and CompuServe workers would often say "Choosy developers choose GIF," spoofing this brand's television advertisements.
An alternate pronunciation with a hard "G" is in expansive usage. The American Heritage Dictionary cites both, recognizing "jif" as the primary pronunciation, while the OED and the Cambridge Dictionary of American English proffer only the /ˈɡɪf/ pronunciation. Merriam-Webster's Collegiate Dictionary cites both pronunciations but places "gif" in the evasion position. The New Oxford American Dictionary gave only "jif" in its 2nd edition but updated it to "jif, gif" in its 3rd edition.
GIFs are convenient for sharp-edged line art with a restricted number of colors. This takes advantage of the format's lossless compression, which supports flat areas of uniform color with well-defined ends.
Empty.gif in a hex editor Practically, a GIF file illustrates a fixed-sized graphical field inhabited with zero or more "images". Various GIF files have a sole image that fills the whole logical screen. Others split the logical screen into distinctive sub-images. The images may also be used as animation frames in an animated GIF file.
GIF files commence with a fixed-length header presenting the version, followed by a fixed measurement Logical Screen Descriptor showing the size and other features of the logical screen. The display descriptor may also stipulate the existence and size of a Global Color Table, which follows next if present.
An image starts with a fixed-length Image Descriptor, which may stipulate the existence and size of a Local Color Table. The image data follows: one byte showing the bit width of the unencoded symbols, followed by a connected list of sub-blocks containing the LZW-encoded data.
GIF is palette-based: the colors utilized in an image in the file contain their RGB values defined in a palette table that can store up to 256 entries. The data for the image denote the colors by their indices (0–255) in the palette table. The color descriptions in the palette can be defined from a color space of millions of shades, but the extreme number of colors a frame can utilize is 256. This restriction seemed sensible when GIF was invented as few people could meet the expense of the hardware to exhibit more colors instantaneously. Easy graphics, cartoons, grey-scale photographs and line drawings typically need fewer than 256 colors.
Every frame can indicate one index as a "transparent background color": any pixel designated this index takes on the color of the pixel in a similar point from the background, which may have been ascertained by a former frame of animation.
Various techniques, collaboratively referred to as dithering, have been invented to estimate a broader range of colors with a minor color palette by making use of pixels of two or more colors to estimate in-between colors. These systems sacrifice spatial resolution to estimate in-depth color resolution. While not portion of the GIF specification, dithering can of course be utilized in images consequently encoded as GIF images. This is not every so often solution for GIF images, both as the loss of spatial resolution generally makes an image look fuzzy on the screen, and because the dithering patterns often intervene with the compressibility of the image data, working against GIF's main purpose.