15/16-bit color (High Color)
24-bit color (True Color)
30/36/48-bit color (Deep Color)
RGB color model
Highcolor graphics (variously spelled Hicolor, Hi-color, Hicolour, and Highcolour, and known as Thousands of colors on a Macintosh) is a method of storing image information in a computer's memory such that each pixel is represented by two bytes. Usually the color is represented by all 16 bits, but some devices also support 15-bit highcolor.
More recently High Color has been adopted to distinguish display systems that can make use of more than 8-bits per color channel (10:10:10:2 or 16:16:16:16 rendering formats) from traditional 8-bit per color channel formats. This is a distinct usage from the 15-bit (5:5:5) or 16-bit (5:6:5) formats traditionally associated with the phrase hi-color.
In 15 bit highcolor, one of the bits of the two bytes is ignored or set aside for an alpha channel, and the remaining 15 bits are split between the red, green, and blue components of the final color, like this:
Sample layout of a 15 bit color data in a 16 bit pixel (in RGBAX notation)
Sample layout of a 15 bit color data and Alpha channel in a 16 bit pixel (in RGBAX notation)
Each of the RGB components has 5 bits associated, giving 25 = 32 intensities of each component. This allows 32,768 possible colours for each pixel.
The popular Cirrus Logic graphics chips of the early 1990s made use of the spare high-order bit for their so-called "mixed" video modes: with bit 15 clear, bits 0 through 14 would be treated as an RGB value as described above, while with bit 15 set, bit 0 through 7 would be interpreted as an 8-bit index into a 256-color palette (with bits 8 through 14 remaining unused.) This would have enabled display of (comparatively) high-quality color images side by side with palette-animated screen elements, but in practice, this feature was hardly used by any software.
When all 16 bits are used, one of the components (usually green, see below) gets an extra bit, allowing 64 levels of intensity for that component, and a total of 65,536 available colors.
Sample layout of real 16 bit color data in a 16 bit pixel (in RGBAX notation)
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