Thresholded undercolor removal and black replacement in a thermal-inkjet

Abstract

Gray component or undercolor in an input image is quantitatively determined, and reduced in amounts nonlinearly related to the quantity of undercolor. The reduced and added signals are applied to control printing by, preferably, scanning inkjet printheads. The total undercolor inking is a substantially monotonic function of the quantity of undercolor. Preferably the reducing and adding are performed in a subtractive-colorant color space, e.g. CMYK space. Preferably image data is initially accepted or generated in terms of additive RGB signals, and these are converted into the subtractive domain. Preferably the reducing and adding means operate only above an undercolor threshold of about twenty to thirty-five percent; and total inking is made between one hundred fifty and two hundred fifty percent when the signals are all gray component. Initial very gradual increases from a threshold are important, for consistent, smooth, regularized dynamic-response in tonal gradations. Preferably these increases follow a spline function. The system works in virtually the same way for black input as chromatic input. Corresponding method and apparatus modules characterize the invention. Automatic operating instructions for the apparatus (or method) modules are stored. Results include minimizing ink usage, consistent with an optimal black, and nearly linearizing composite grays.