Method and apparatus for color halftoning using interlocked threshold

Abstract

A dispersed-dot stochastic dither array is provided for rendering halftone images having excellent visual quality and are created by a minimum density variance method. By minimizing the variance in the number of dots within each local region of the image, a smooth and dispersed distribution of dots may be obtained. For color printing, a separate threshold array is generated for each of the color planes, however, the stochastic screens are interlocked so that the threshold arrays are generated while considering the other color threshold arrays. In this manner, a blue noise distribution may be produced by the individual arrays as well as by any combination of the individual arrays. When generating a single threshold array for a color plane, a particular criterion is used to determine where the next threshold value should be located, and the selection of a threshold location in each array considers the criterion for all the threshold arrays being generated. This may reduce the quality of an individual array by a small amount, but it allows for the combination of the arrays to have a significantly better distribution. The simplest approach uses the average of the criteria for all threshold arrays, however, to improve the quality of the individual arrays, the preferred method of the present invention uses a weighted average. When selecting a threshold location for an array 'i' using weighted averages, the criterion associated with array i may be assigned a greater weight than each of the other individual arrays' criteria. By using the interlocked stochastic screens approach, the overall visual affect will be improved for both individual threshold arrays and for a combination of more than one of the threshold arrays. The interlocked approach is clearly superior for lighter shades, given any number of color planes.