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Concept
Indexed color
Résumé
In computing, indexed color is a technique to manage s' colors in a limited fashion, in order to save computer memory and file storage, while speeding up display refresh and file transfers. It is a form of vector quantization compression.
When an image is encoded in this way, color information is not directly carried by the image pixel data, but is stored in a separate piece of data called a color lookup table (CLUT) or palette: an array of color specifications. Every element in the array represents a color, indexed by its position within the array. Each image pixel does not contain the full specification of its color, but only its index into the palette. This technique is sometimes referred as pseudocolor or indirect color, as colors are addressed indirectly.
Early graphics display systems that used 8-bit indexed color with frame buffers and color lookup tables include Shoup's SuperPaint (1973) and the video frame buffer described in 1975 by Kajiya, Sutherland, and Cheadle. These supported a palette of 256 RGB colors. SuperPaint used a shift-register frame buffer, while the Kajiya et al. system used a random-access frame buffer.
A few earlier systems used 3-bit color, but typically treated the bits as independent red, green, and blue on/off bits rather than jointly as an index into a CLUT.
Palette (computing)
The palette itself stores a limited number of distinct colors; 4, 16 or
256 are the most common cases. These limits are often imposed by the
target architecture's display adapter
hardware, so it is not a coincidence that those numbers are
exact powers of two (the binary code):
22 = 4,
24 = 16 and
28 = 256. While 256 values can
be fit into a single 8-bit byte (and then a single indexed color
pixel also occupies a single byte), pixel indices with 16 (4-bit, a
nibble) or fewer colors can be packed together into a single byte
(two nibbles per byte, if 16 colors are employed, or four 2-bit pixels
per byte if using 4 colors).
Source officielle
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