Black Generation
Overprinting CMY generally doesn’t produce sufficiently high ink density in shadows. To increase the tonal range in shadows, some black is added.
There are two methods of black generation, undercolor removal (UCR) and gray component replacement (GCR).
In most color reproductions the image contains some amounts of all three primaries (RGB or CMY). In the absence of black a CMY separation can be determined.
- At any given point in the image, one or two primaries dominate and determine the hue.
- The remaining one or two primaries may be thought of as determining the lightness (or darkness).
- This effect is the same as adding gray to the dominant hue and hence could be equally well obtained by adding a percentage of black.
- This is the principal of UCR and GCR.