Research On Digital Halftoning And Watermarking Techniques For Images

Digital halftoning is to quantize a continuous-tone image to a binary image or a color image with limited colors that the output halftoned image looks like the original multitone image when viewed from a distance. Halftone images routinely appear in low-resolution facilities, such as laser and inkjet printers, facsimile machines and low-cost liquid crystal displays. It can also be applied in mobile equipments, such as cellphones and PDAs.Halftone image is commonly applied in the industries of newspaper & magazine publication, identification, currency, check or confidential documents. It would be one of the most important means of exchanging information in the foreseeable future. Since the internet is an open network infrastructure, it is indispensable to employ effective protection mechanisms, for example the watermarking techniques, to protect halftone images from being attacked when transmitted along the network. Digital watermarking is a process to embed sign information (watermark) into a digital signal, such as digital image, speech, text document, book or video, to protect the ownership of the digital information, to prevent the illegal use without permission, and to authenticate the integrity of the content by examining the existence of the watermark.The key problem of digital halftoning is how to arrange the pixels of the output halftoned image with a limited scope of a gamut. Several halftone techniques for graylevel images are introduced in this paper, such as ordered dithering, direct binary search, standard error diffusion and adaptive error diffusion, as well as a color error diffusion halftoning system. The performances of the presented algorithms are evaluated by comparing the computational complexity and the visual effect. Furthermore, this paper presents some new performance criterions for halftone algorithms.We also study some representative watermarking technologies for halftone images and quantitatively analyze the robustness and payload of these algorithms. From experimental results, we find that, comparing with a halftoned image without a watermark, the visual perception of the output halftoned image with a watermark is very different from the original continuous-tone image. And these watermarking methods could only resist some mild attacks but be vulnerable to other vicious attacks. It would be one of the unremitting research works to seek watermarking methods for halftoned images that could resist as many attack algorithms as possible.This paper proposes a new evaluation scheme to assess the halftone qualities of different error diffusion halftoning algorithms. An integer-coefficient filter bank is applied to every error diffusion halftone image, and the halftone image is decomposed into 16 subbands by using low-pass filters and high-pass filters along the horizon and vertical orientations respectively. Based on these subband images, high-order distribution statistics are defined to evaluate the qualities of different error diffusion filters. The correlations between the subbands of the residual image and the original image are calculated to measure the distortion caused by the error filter in error diffusion halftoning. In order to evaluate the qualities of different error diffusion filters, we use a constant-tone patch to be halftoned by three commonly used error diffusion filters (Floyd-Steinberg, Jarvis and Stucki) and the energy distribution of the patch generated by Jarvis error filter is most similar to the normal distribution among these three error filters. And our experimental results of correlation are consistent with the previous research works.Finally this paper discusses the future research works and the potential application of digital halftoning and the watermarking technologies.