Method for halftoning using a difference weighting function

Abstract

An improved method of digital halftoning using “screens” or dither arrays is provided, including stochastic dither arrays (e.g., stochastic screening). The weakness of ordinary stochastic dithering is its isolated or dispersed dots and the related problems of dot gain and consistent dot formation when used with certain types of printers. However, the inconsistent formation of isolated dots causes unpredictable variations in uniformity and tone. The present invention solves this problem by grouping dots into small clusters, which may be rendered more consistently. Unlike conventional clustered-dot halftoning, however, this method arranges the dot clusters in a stochastic fashion to avoid objectionable periodic artifacts. A novel weighting function is used to generate the dot clusters, in which one function of a first extent is subtracted from another function of a second extent, thereby creating clusters of dots that center at locations where the weighting function chooses to place dots. Locations very close to an existing dot will have an increased chance of being selected; at a certain distance away from the existing dot, where the new weighting function reaches a maximum (or maxima), locations will have a smaller chance of being selected; once past this distance, the locations will have a gradually increasing chance of selection.