Isotropic error diffusion halftoning

The inherently causal nature of conventional single-pass error diffusion (ED) halftoning results in asymmetric diffusion of error. This results in the introduction of directional artifacts in the output halftone. In this paper we propose a novel two-pass algorithm which achieves symmetric error diffusion by using a zero-phase signal transfer function. We determine conditions under which isotropic diffusion of error and noise suppression are achieved. Experimental results demonstrate that the proposed algorithm breaks up worms and randomizes their direction, thus making the output halftone more visually appealing as compared to conventional error diffusion.