Techniques for Coding Dithered Two-Level Pictures

Techniques for representing the entire gray scale of a picture by only two levels have been receiving considerable attention 1 - 1 0 because many devices are limited to recording or displaying two-level signals. Although these techniques may differ in specific algorithms, they provide the subjective illusion of a wide range of gray shades by controlling the proportion of picture elements in a neighborhood t h a t are in the "on" state. One of the techniques (called "dither" and studied in detail by Limb, 2 Lippel and Kurland, 3 and Judice et al. 9-10 ) consists of comparing the multilevel input image signal with a position-dependent set of thresholds and setting only those picture elements to "white" (or 1) where the image 809 PEL COLUMN I £+1 £+2 £+3 0 192 128 64 32 160 LINE m 224 96 LINE m + 1 48 176 16 144 LINE m +2