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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 [27]:

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

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:

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

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.



David Blythe
1999-08-06