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

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 in the unused bits, instead of zeroes. So, autodetecting STE palettes may not be possible to do in a completely reliable way.)

Images with 12-bit palettes

 * bwlady.pc1
 * clinton.img

Other

 * bilboule.pi1 - Example of a file that appears to have garbage in the unused palette bits

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

Software
Software known to support 12-bit STE-style palettes:
 * RECOIL
 * Deark