Error diffusion techniques with reduced patterning artifacts and directional hysteresis

Abstract

Digital halftoning is the process of transforming a continuous-tone image to a pattern of black and white pixels that, due to lack of high frequency resolution of naked eyes, has the appearance of the original continuous-tone image. The term spatial dithering is also used to refer to this process. It is necessary for displaying grayscale images when direct rendition of gray tones is impossible. The most common example is to print grayscale images on paper using a bilevel printing device. Nowadays, among the many available digital halftoning schemes, error diffusion is believed to be one of the most effective approaches that can provide the best quality. However, several objectionable patterning artifacts and directional hysteresis are unavoidable in conventional error diffusion schemes since sequential predetermined order is required to diffuse the quantization error. In this thesis, two digital halftoning methods are proposed for improving the visual appearance of the halftoned output by reducing the patterning artifacts and directional hysteresis. First, a new approach of digital halftoning is proposed to diffuse errors with a more symmetric error distribution by making use of the concept of delayed decision. From simulation results, it shows that the diffusion performance is improved by effectively reducing the patterning artifacts of the diffusion outputs. Second, a new halftoning method based on multiscale error diffusion is proposed. The proposed method can definitely improve the diffusion performance by effectively removing patterning artifacts and eliminating the boundary and "blackhoie" effects. It is superior to conventional error diffusion methods in a way that directional hysteresis can be avoided since no sequential predetermined order is required. Corresponding dot-overlap effect compensation schemes for the two newly proposed halftoning methods are also introduced. By making use of a circular dot-overlap printer model, the compensation scheme can remove the dot-overlap distortion when the methods are used to generate haiftoned output for printing devices.