Rank-Ordered Error Diffusion: Method and Applications

摘要

We present a specialized form of error diffusion that addresses certain long-standing problems associated with operating on images possessing halftone structure as well as other images with local high contrast. For instance, when rendering an image to printable form via quantization reduction, image quality defects often result if that image is a scanned halftone. Rendering such an image via conventional error diffusion typically produces fragmented dots, which can appear grainy and be unstable in printed density. Rendering by simple thresholding or rehalftoning often produces moire, and descreening blurs the image. Another difficulty arises in printers that utilize a binary image path, where an image is rasterized directly to halftone form. In that form it is very difficult to perform basic image processing operations such as applying a digital tone reproduction curve. The image processing operator introduced in this paper, rank-order error diffusion (ROED), has been developed to address these problems. ROED utilizes brightness ranking of pixels within a diffusion mask to diffuse quantization error at a pixel. This approach to diffusion results in an image-structure-adaptive quantization with many useful properties. The present paper describes the basic methodology of ROED as well as several applications.