A Dot Placement Approach to Stochastic Screening Using Bitmasks

Abstract

FM or stochastic screening is a popular approach to halftoning for many applications. The error diffusion algorithm delivers extremely good screen quality but at the price of a computationally-intensive runtime process. Point processes, using either dither arrays or bitmask sets, have efficient runtime requirements but often produce halftones of much lower quality. The generation of such screens usually involves starting with a random pattern and applying some simulated annealing process to gradually improve its characteristics. This paper proposes a method for generating stochastic patterns that employs a dot placement algorithm in which each dot is placed in a position “appropriate” for producing good stochastic output. The algorithm is then enhanced by applying a smoothing step at the end of each halftone pattern generation to adjust any dots that, due to the placement of later dots, are now in sub-optimal positions. Although the algorithm can be used to produce dither arrays, it is primarily aimed at generating bitmasks where the additional degree pattern freedom is exploited to improve pattern smoothness. The algorithm also permits second order stochastic patterns for use with imprecise print devices such as electro-photographic printers.