Combining error diffusion, dithering, and over-modulation for smooth multilevel printing

Multilevel halftoning (multitoning) is an extension of bitonal halftoning, in which the appearance of intermediate tones is created by the spatial modulation of more than two printing states, i.e., black, white, and one or more shades of gray. Conventional multitone schemes using error diffusion halftoning algorithms normally generate contouring artifacts at regions where the code values of the original image are close to those of the multilevel printing states (or output gray levels). A distinct texture transition also typically occurs near these gray levels. In a previous study, an over-modulation algorithm was proposed to improve the texture transition at those regions for the ease of multilevel dithering. In this study, a new algorithm will be proposed that combines error diffusion, blue-noise dithering, and over-modulation so that high-quality multilevel printing with smooth texture transition can be achieved.