Threshold-diffuse hybrid half-toning methods

Gray-scale images can be displayed on binary display devices using a process known as half-toning where gray intensities are approximated by different distributions of black and white pixels. There is a significant number of methods with which these binary or half-toned approximations can be generated. Most of the available methods can be categorized as either a threshold-matrix half-toning method or as an error-diffusion half-toning method. The threshold-matrix half-toning methods are fast and simple to implement and they can be used with or without an increase in resolution. The binary approximation of each gray-scale value is computed independently. Images half-toned with threshold matrices often exhibit quantization bands in areas of the image where a smooth intensity change occurs. Error-diffusion half-toning methods approximate the image one segment at a time. Any error resulting from a segment's approximation is propagated to unprocessed segments. In this paper we present some results from our current research focused on producing half-toning methods that are hybrids of threshold-matrix and error-diffusion half-toning. In particular, we introduce three hybrid methods: Threshold-matrix with error-diffusion, Variable shape threshold-matrix, and Variable threshold with error-diffusion. Each of these techniques brings together aspects from both threshold-matrix and error-diffusion half-toning methods. The common feature of these three methods is that they remove the quantization bands introduced by the use of small threshold matrices.