A Fast Linking Approach for CMYK to CMYK Conversion Preserving Black Separation in ICC Color Management System

摘要

In the linking step of the standard ICC color management workflow for CMYK to CMYK conversion, a CMM takes an AToBn tag (n = 0, 1, or 2) from a source ICC profile to convert a color from the source color space to PCS (profile connection space), and then takes a BToAn tag from the destination ICC profile to convert the color from. PCS to the destination color space. This approach may give satisfactory result perceptually or colorimetrically. However, it does not preserve the K channel for CMYK to CMYK conversion, which is often required in graphic ait's market. The problem is that the structure of a BtoAn tag is designed to convert colors from PCS to a device color space ignoring the K values from the source color space. Different approaches have been developed to control K in CMYK to CMYK printing, yet none of them well fits into the "Profile - PCS - Profile" model in the ICC color management system. A traditional approach is to transform the source CMYK to the destination CMYK by 1.-D TRC curves and GCR/UCR tables. This method is so simple that it cannot accurately transform colors perceptually or colorimetrically. Another method is to build a 4-D CMYK to CMYK closed-loop lookup table (LUT) (or a deviceLink ICC profile) for the color transformation. However, this approach does not fit into opened color management workflows for it ties the source and the destination color spaces in the color characterization step. A specialized CMM may preserve K for a limit number of colors by mapping those CMYK colors to some carefully chosen PCS colors in both the AToBi tag and the BToAi tag. A more complete solution is to move to smart linking in which gamut mapping is performed in the real-time linking at a CMM. This method seems to solve all problems existed in the CMYK to CMYK conversion. However, it introduces new problems: 1) gamut mapping at real-time linking is often unacceptable slow; 2) gamut mapping may not be optimized or may be unreliable; 3) manual adjustment for building high quality maps does not fit to the smart CMM workflow. A new approach is described in this paper to solve these problems. Instead of using a BtoAn tag from the destination profile for color transformation, a new tag is created to map colors in PCS (L*a*b* or XYZ) with different K values to different CMY values. A set of 3-D LUTs for different K values are created for the conversion from PCS to CMY, and 1-D LUTs are created for the conversion from luminance to K and to guide a CMM to perform the interpolation from KPCS (K plus PCS) to CMYK. The gamut mapping is performed in the step to create the profile, thus avoiding realtime gamut mapping in a CMM. With this approach, the black channel is preserved; the "Profile - PCS - Profile" approach is still valid; and the gamut mapping is not performed during linking in a CMM. Therefore, gamut mapping can be manually adjusted for high quality color mapping, the linking is almost as easy and fast as the standard linking, and the black channel is preserved.