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In printing, most color images can be
reproduced using only four colors of ink or toner: the so-called
"process" colors C (cyan), Y (yellow), and M (magenta), plus K (black).
Color separation —breaking
down original images into these four component colors—takes place
during prepress. Each separation is a record of the C, M, Y, or K
information in the original image; and each corresponds to a piece of
film, a plate, and a color of ink on the press. (The film will not be
present in computer-to-plate, computer-to-press, and digital print
workflows. Digital printing also eliminates the plates, but the
principles of color separation remain the same no matter what method is
used.)
Separations are produced by photographing or digitally
exposing the original image through successive filters of the additive
primary colors R (red), G (green), and B (blue). Each filter creates a
record of the additive color's subtractive complement: red yields the
cyan separation, green the magenta separation, and blue the yellow
separation. An additional, modified filter records the black
separation. When overprinted, the separations recombine to re-create
the hues and tones of the original (to the extent that the color gamut of the printing device will
permit).
The diagram above shows the C, M, Y, and K separations
individually and in the combinations CM, CMY, and CMYK. In proofing,
the colors may be presented in this way as a set of hard-copy
"progressives" showing the results of the overprint combinations.
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