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Summary:
We already mentioned that the principle of color reproduction by superimposing halftone dots in the printing process was essentially a subtractive mixture, and that the correction of optical dot gain was needed. In this paper, we present the method of outputting the C, M, Y and Bk dot areas converted from the image signal harvested from a continuous tone color original by a scanner using proportional tone compression (Numakura-Yamatoya equation). The Numakura-Yamatoya equation is based on the Yule-Nielsen equation in exponential form a = 1-10-Da/n over 1-10-Ds/n, but the term Da in the equation is replaced to the term K * Ds in the Numakura-Yamatoya equation. The coefficient K works as a tone compressive function in the case of using ideal printing inks that do not have extra light absorbance. We fixed on the dot area of black by gray component replacement and we determined other C, M, Y dot areas so as to equalize the quantity of light through a color filter (R, G, B). These equations are constructed with the Pollak equation containing our corrective terms of optical dot gain. The method of determining the dot areas is to solve simultaneous quadratic equations with three unknowns by using successive approximation. This conversion contains color balance, dot gain (optical and mechanical), GCR and masking (color correction). So far these processes have been dealt with empirically. While LUT is one of the empirical methods, the proposed treatment is to construct a numerical model of color images made up of superimposed dot areas, and fractional dot areas required are calculated numerically.