15-bit High Color
In 15 bit high color, 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 colors 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.
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