13.2.4.6 CMY Conversion

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 [30]:

$$\left[ \begin{array}{c} C \\ M \\ Y \end{array}\right] = \... ...ht] - \left[ \begin{array}{c} R \\ G \\ B \end{array}\right]$$

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:

$$\left[ \begin{array}{c} C \\ M \\ Y \\ 1 \end{array}\righ... ...\left[ \begin{array}{c} R \\ G \\ B \\ 1 \end{array}\right]$$

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.