Color FM screen design using DBS algorithm

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Abstract
When halftoning a color image for a bi-level color printer, one has to obtain the halftones of the cyan, magenta, and yellow planes if the printer is a three color device. For a four-color printer, one has to also obtain the halftone of the black plane. Suppose a source color images is represented by the red, green, and blue components. The simple way of halftoning a color image using a dither matrix is to halftone each color plane independently using the same matrix. This will result in halftone dots of different colors overlapping each other, thus increasing the graininess of an image. Simple schemes such as shifting the matrices have been proposed in the past, but they usually reduce the dot overlap at the cost of increasing other artifacts, such as fuzziness. We propose an algorithm to jointly design a set of dither matrices such that the overall graininess is minimized. We use the direct binary search (DBS) algorithm to design a dither matrix for each of the primary colors of a printer, cyan, magenta, yellow, and black. A color fluctuation function is defined for the halftone patterns of a set of constant tone color patches in a uniform color space such as CIEL*a*b*. The color fluctuation function is then minimized on a level-by-level basis using swap operations. Efficient evaluation of the color fluctuation function allows the optimization to converge at a reasonable speed. We show that we are able to achieve halftone image quality comparable to that of the direct binary search (DBS) algorithm at a significantly lower computational cost. Because the dither matrices are pre-computed, efficient implementation in either hardware or software is possible.

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