M4.19 HALFTONE PATTERNS FOR ARBITRARY SCREEN
T. Srinivasa Rao, Gonzalo R. Arce · 1988
Ordered dither has long been considered to be a simple and effective method of image binarization. If ft is the two dimensional continuous signal to be halftoned, then ordered dither consists of thresholding the discrete space signal fm = fvm against a periodic spatial screen am whose periodicity is defined by a periodicity matrix N. Although most dithering schemes used in practice do not assume N to be a diagonal matrix, most theory on dithering has done so. Consequently, most optimal dither patterns have been designed for rectangularly periodic screen functions. In this paper the benefits of altering N will be analyzed in an effort to improve the ordered dither algorithm. Optimal dither patterns for several representative non-rectangular screen periodicities will be derived. It will be shown that the optimal pattern sequences for these periodicities have superior gray scale rendition capabilities than previously considered dither patterns resulting in substantially improved image fidelity.