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Managing Color With ColorSync


CMY-Based Color Spaces

CMY-based color spaces are most commonly used in color printing systems. They are device dependent and subtractive in nature. The groups of color spaces within the CMY family include

The name CMYK refers to cyan, magenta, yellow, and key (represented by black). Cyan, magenta, and yellow are the three primary colors in this color space, and red, green, and blue are the three secondaries. Theoretically black is not needed. However, when full-saturation cyan, magenta, and yellow inks are mixed equally on paper, the result is usually a dark brown, rather than black. Therefore, black ink is overprinted in darker areas to expand the dynamic range and give a better appearance. Printing with black ink makes it possible to use less cyan, magenta, and yellow ink. This may prevent saturation, especially on materials such as plain paper which cannot accept too much ink. Using black can also reduce the cost per page because cyan, magenta, and yellow inks are generally more expensive than black ink. It can also provide a sharper image, because a single dot of black ink is used in place of three dots of other inks.

Figure 1-4 shows how additive and subtractive colors mix to form other colors.

Figure 1-4 Additive and subtractive colors

Theoretically, the relation between RGB values and CMY values in CMYK space is quite simple:

Cyan        = 1.0 - red
Magenta     = 1.0 - green
Yellow      = 1.0 - blue

(where red, green, and blue intensities are expressed as fractional values varying from 0 to 1). In reality, the process of deriving the cyan, magenta, yellow, and black values from a color expressed in RGB space is complex, involving device-specific, ink-specific, and even paper-specific calculations of the amount of black to add in dark areas ( black generation) and the amount of other ink to remove ( undercolor removal) where black is to be printed. Therefore, when ColorSync converts between CMYK and RGB color spaces , it uses an elaborate system of multi-dimensional lookup tables, which ColorSync knows how to interpret. This information is stored in profiles, which are defined in the section Color Conversion and Color Matching .


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