Modeling a CMYK printer as an RGB printer

In CMYK output devices, many colors are reproducible by more than one combination of CMYK colorants. When a CMYK device is modeled as an RGB device, each RGB combination must produce a unique CMYK combination. Under color removal (UCR) and gray component replacement (GCR) techniques have been traditionally used to calculate these unique combinations. These techniques are simple to implement, but cannot fully utilize the color gamut possible with the additional K colorant. Other brute-force techniques that search the entire CMYK signal space for desirable combinations produce good results but are unsuitable for real-time implementation. In this paper, we introduce a flexible computational structure for converting RGB or CMY signals to CMYK signals. This structure, which can be viewed as an extension of the traditional UCR and GCR techniques, uses multiple sets of 1-D CMYK look-up tables (LUTs) to control the CMYK colorant usage. The LUTs are strategically placed on the center diagonal and boundaries of the input signal cube. By properly designing these LUTs, we obtain a model for RGB-to-CMYK conversion that utilizes most of the available CMYK gamut and also corrects certain non-ideal device behaviors, such as hue shifts along lines from pure colors to black or white.