Research On Key Technology Of Image Hybrid Halftoning For Color Printing

A digital image should be converted to the image of the same depth as the depth of image output device. To halftone an image is to convert a high depth image into a low depth image, so as to make the low depth image visually similar to the original one. Halftone technology can be applied extensively in many fields related to image output, such as printing, out-door advertisement billboard, television, video game, animation and on-line image display and transfer etc, where halftone is the key technology to determine quality of output image. Halftone technology is of great significance to the control and optimization of output image quality, output image color predicion and image quality evaluating methods applied to different output devices. .In the practice of halftoning image, human factors in output image quality evaluation, performance limitation and versatility of image output devices pose great challenges to the research and application of halftone technology. Aiming at improving output image quality, and taking color printing as application background and image halftone and its related technology as main topics, this thesis carries out a series of in-depth research, including the design method of hybrid dithering matrix based on geometry subdivision, color shift of hybrid halftone image overlay and halftone image quality evaluation system.The main contributions of this paper can be summarized as follows:(1) develops an optimized method for the geometry subdivision in order to reduce the great disparity between highlight and shadow clustered dot density of the hybrid dithering matrix based on the geometry subdivision. The unbalance of clustered dot density will limit the increase of the clustered dot density, while clustered density is crucial to high quality image print. Two theorems are deducted from Euler formula in the dissertation. Theorem of all-triangle geometry subdivision can be used to explain why big difference between highlight and shadow clustered dot density is made in the existing research. Based on theorem of all-bilateral geometry subdivision, an algorithm is developed to optimize the geometry subdivision. In the experiment of the optimization algorithm, the ratio between vertex and faces of the subdivision is improved to above 0.9 from 0.5. In the hybrid dithering matrix designed with the optimized subdivision, the ratio between highlight and shadow clustered density is also improved to above 0.9 from 0.5.(2) proposes a method to construct dithering matrix with equal clustered dot density by expanding the set of clustered dot central points and forming an all-bilateral subdivision with the clustered dot centers as the vertex. With dithering matrix of equal clustered dot density, higher clustered dot density and higher image print quality can be achieved. To justify the method of expanding the set of clustered dot central points, an experiment is designed to compare the evenness of the images of expanded central points and unexpanded one. In addition, a comparison is made between the quality factor of bilateral of subdivisions based on expanded central points and the original one. The result of the experiment shows: the PSNR of the images of expanded central points and original one is approximate equal.(3) presents a method to calculate color shift by simulating misalignment of color separations with halftone image overlay, Yule-Nielsen Neugebauer color equation and offset printing color standard ISO 12647. By comparing the color shift of amplitude modulated halftone and hybrid halftone developed in this research, the simulation experiment concludes that: color shift of the hybrid dithering matrix designed with three different random seeds can meet the requirement of printing procedure.(4) finally, establishes a unified halftone image quality evaluation approach based on non-ideal printer model by learning from the existing halftone image quality evaluation model based on the ideal printer model. By adopting mathematic morphology's Dilate operation on halftone image and filtering small clustered dot algorithm, dot gain of offset printing is simulated and resulted dot gain curve fits the known standard one. By calculating human perceived images with HVS filter, an image quality evaluation test is conducted with approaches based on both non-ideal printer model and the ideal printer model. The result of the experiment indicates that the halftone image quality evaluation approach based on the non-ideal printer model fits practical offset printing better.