Color bitmap merging of black objects using separation maximum value clipping

Abstract

A bitmap-based digital color printing method and system is described which automatically detects when a black object is to be printed and changes the Bit Block Transfer (BitBlt) method so that the black color becomes a mixture of black plus the background colorants, limited by an allowed maximum non-black value for each separation. More specifically, the process determines, for each non-black separation, a maximum allowable amount to be used in black printing, reads the existing background color bits, and clips them if necessary to the allowed maximum before mixing them into the black color. The result will be a limitation on the total colorant amount of the black area, which improves some printer defects caused by too much colorant. The clipping can be accomplished at the bit level during the read-modify-write process if both the maximum and actual colors are represented with the same monotonic halftone dot specification, and are ANDed together to form the clipped bits. For many printing systems the resulting prints exhibit fewer visual defects. Single-color black objects printed as clipped process black exhibit greater density and gloss and create fewer printer defect problems such as misregistration and xerographic "tenting". Conversely, process black objects on white are printed as single-color black objects and do not exhibit objectionable problems such as color fringing, excessive colorant, and excessive gloss contrast with the background.

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