AM-FM hybrid color screen design to reduce brightness variation

AM-FM hybrid screen represents a clustered dot halftone screen whose dot clusters are aperiodic and dot growth pattern is irregular. Unlike AM ordered screen, AM-FM hybrid screen is free of the inter-screen and subjective moirés. However, it results in brightness variation called as stochastic noise. In this paper, a new AM-FM hybrid screen design technique is proposed. In the proposed method, locations of dot cluster centers are determined first by a distance measure based algorithm. They serve as initial patterns to construct AM-FM hybrid screens. Number of dot cluster centers is chosen based on the maximum screen frequency allowed for a given printer. Dot cluster centers should be distributed homogeneously within and between color channels. Also, there should not be noticeable tiling artifacts. Dot orders of highlight area are decided during the construction of initial patterns. For the gray levels darker than that of initial patterns, dot clusters start to be formed. In order to expand dot clusters while maintaining the green noise characteristics with a peak at the principal frequency of cluster centers, an optimum dot growth filter is designed. A set of pixel locations is determined by the proposed dot growth filter. In order to reduce brightness variation and low frequency noise, channel dependent dot growth algorithm is proposed. Among the set of candidates, a pixel location that minimizes brightness variation is selected.