The Research Of Digital Halftone Technology

RIP(Raster Image Processor) software is very important for the performance of inkjet printer. Digital halftoning is one of the key technologies of the RIP software,directly determines output image quality, so its position in the printing system is very important.The main contant of this thesis is two parts: grayscale halftoning and color halftoning.In grayscale halftoning, the conventional digital halftone algorithms are introduced firstly. On the basis of analysis of their defects and the reasons of causing the printer's distortion, several improved algorithms are introduced, such as dot diffusion, blue noise error diffusion, edge enhancement error diffusion and inkjet printer model-based halftoning, which improve the defects of classical error diffusion such as parallelism, artifacts at certain intensity levels, blurred edges and grayscale distortion respectively. Finally, these algorithms are compared detailedly to give reference to the design of RIP software in printer.In color halftoning, vector error diffusion is emphasized in this thesis. Compared with conventional color halftoning method, it exploits the correlation between colorant planes and renders superior halftone images. In this thesis, the Minimal Brightness Variation Criterion is introduced in vector error diffusion, then a Vector Error Diffusion Based on the Minimal Brightness Variation Criterion is presented. Fistly, the Minimal Brightness Variation Criterion is introduced into the quantization process to determine a Minimal Brightness Variation Quadruple of an input RGB triplet; Secondly, the output color is determined by traversing a decision tree of a MBVQ; Thirdly, an optimal error filter is generated, so that quantization error is diffused to frequencies and colors which the Human Visual System is least sensitive to. It is proved that the halftone colors used to render any input color are reduced from the original eight colors to four colors whose brightness variation is minimal, which makes the halftone image has better visual effect.