Research On Digital Halftoning Technique Based On Error Diffusion

Digital halftone technique, which basis on human visual characteristics and color characteristics of image, is the tone rendition technology of transforming an image with greater amplitude resolution to one with lesser amplitude on two levels or on finite levels by mathematics and computer. The technology has widely applied to printing technology, compression memory of digital image, transmission of digital image textile, typography and medicine etc. The research of digital halftone technique is practical significance and application value.After discussing the advantages and disadvantages of the current digital halftone algorithms, this thesis studies digital halftoning technique mainly aimed at gray image. The main contents include: after analyzing the structure of image reconstruction system, then the basic principles, classifications of the digital halftone technology and the human visual system (HVS) model are studied. We focused mainly on digital halftone technique and error diffusion algorithm related idea, principle and three factors impacting digital halftone technique– the choice of image scanning path, threshold and the coefficients of error filter.Two halftone algorithms are proposed in this paper.1. An improved error diffusion algorithm based on directional correlation. In this algorithm, the image is divided into background regions and edge regions according to the image gradient. The standard F-S error diffusion algorithm is applied to the background regions, and correlation-based algorithm is used for the edge regions in the halftoning process. Based on the differences of the vertical, horizontal and two diagonal between pixels, the biggest difference is assigned the smallest weight and the smallest difference is assigned the biggest weight. This algorithm not only maintains the advantages of the standard error diffusion algorithm, but also reduces the false contour.2. An ameliorated error diffusion algorithm based on bidirectional feedback and HVS: A feedback coefficient is added to the standard error diffusion algorithm, so it is called bidirectional feedback algorithm. The improved algorithm adds a middle band in the high and low frequency components according to HVS models, and changes error diffusion filer coefficients dynamically based on the characteristics of local gray and HVS feature as well. This algorithm overcomes directional texture effectively, and the halftone image quality of our algorithm is much better than that of the standard F-S algorithm.Finally the author builds a digital halftoning working bench, and presents our experiments based on some error diffusion algorithms. From the results, we can conclude that these two new algorithms display better visual quality than that of others.