The CMYK2 color space

In print production, special transfer functions are applied to tune the conversion results. For example, the Adobe PostScript interpreter introduces the undercolor-removal function fUCR(K) for reducing the CMY components and the black-generation function fBG(K) for controlling the amount of black. Using these functions, the conversion from CMY to CMYK2 ( where ‘2’ stands for distinguishing from the above discussed form) is the following:

K = min(C, M, Y)

C’ =  C - fUCR(K)

M’ = M - fUCR(K)

Y’ = Y - fUCR(K)

K’ = fBG(K)

The functions fUCR and fBG are usually nonlinear. Using the following formula, relatively close result can be produced to the Adobe PhotoShop’s result:

fUCR(K) = sK*K,

fBG(K) = Kmax*(K-K0)/(1-K0)  for K>=K0,  or  =0  otherwise.

The formula’s parameters can be adjusted for improved result - the default settings:

sK = 0.1  ,  K0 = 0.3 , Kmax = 0.9.