Atari ST color palette

The Atari ST computer supports 3 bits per color channel, giving (2^3)^3 = 512 different colors. In the standard video modes, no more than 16 different colors can be displayed at the same time, and a color palette is used to define the available colors.

Format details
In a number of Atari graphics formats, the palette is stored in a format with 16 bits per palette entry, only 9 of which are significant: xx xx xx xx xx R2 R1 R0 xx G2 G1 G0 xx B2 B1 B0

Atari STE palettes
The Atari "STE" models increased the color depth from 3 to 4 bits per color channel, thus supporting (2^4)^3 = 4096 different colors. For compatibility reasons, its palettes have the least significant bit in the position one might otherwise expect to be used for the most significant bit: xx xx xx xx R0 R3 R2 R1 G0 G3 G2 G1 B0 B3 B2 B1

Some Atari image files use these Atari STE 12-bit palettes, usually with no external indication that they do so. Trying to detect and interpret such palettes is a good idea, as grayscale (and grayscale-ish) images can be significantly improved by the extra color depth. (Unfortunately, it seems that some non-STE files contain garbage (or private data) in the unused bits, instead of zeroes. So, autodetecting STE palettes may not be possible to do in a completely reliable way.)

Spectrum 512 Enhanced palettes
For completeness, we mention the obscure Spectrum 512 "Enhanced" format, which extends the palette even further, to 5 bits per color channel: R0 G0 B0 xx R1 R4 R3 R2 G1 G4 G3 G2 B1 B4 B3 B2

Bi-level images
''[TODO: Explain how palettes for bi-level "high resolution" modes should be interpreted (if they should be interpreted at all). In practice, many such palettes contain white and black as expected, but others contain improbable colors such as dark blue and black, or red and white.]''

Images with 12-bit palettes

 * bwlady.pc1
 * clinton.img

Other

 * bilboule.pi1 - Example of a file in which the extra palette bits are not zero

Related formats
Formats that use this style of palette (12-bit palettes are not necessarily allowed for all of these formats):
 * DEGAS image
 * Tiny Stuff
 * NEOchrome
 * At least one of the color GEM Raster formats

Other computer palettes

 * Computer Palettes

Software
Software known to support 12-bit STE-style palettes:
 * RECOIL
 * Tom's Editor Desktop
 * Tom's Editor Desktop