The CMY color space describes colors in terms of the subtractive
primaries: cyan, magenta, and yellow. CMY is used mainly for hardcopy
devices such as color printers. Generally, the conversion from
RGB to CMY follows the equation [18]:
CMY conversion may be performed using the color matrix or a scale and
bias operation. The conversion is equivalent to a scale by -1 and a
bias by +1. Using the 4x4 color matrix, the equation may be restated
as:
Here, the incoming alpha channel must be equal to 1. If
the source is RGB, the 1 will be added automatically in the format
conversion stage of the pipeline.
A related color space, CMYK, uses a fourth channel (K) to represent black. Since conversion to CMYK requires a min() operation, it cannot be performed using the color matrix.
The extension EXT_CMYKA also supports conversion to and from CMYK and CMYKA. This extension is currently supported by Evans & Sutherland.