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 f_UCR(K) for reducing the CMY components and the black-generation function f_BG(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 - f_UCR(K)

M’ = M - f_UCR(K)

Y’ = Y - f_UCR(K)

K’ = f_BG(K)

The functions f_UCR and f_BG are usually nonlinear. Using the following formula, relatively close results can be produced with Adobe PhotoShop:

f_UCR(K) = s_K*K,

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

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

s_K = 0.1  ,  K₀ = 0.3 , K_max = 0.9.