Digital halftoning algorithms for high-quality and high-speed electro-photographic printing.

摘要

The objective of this research is to develop new digital halftoning algorithms targeting high-quality, high-speed electro-photographic (EP) printing for scan-to-print applications. Conventionally, AM halftoning modulates the size of printed dots. It has very low computation load, good print stability for EP printing, but poor spatial resolution and Moire resistance. Alternatively, FM halftoning modulates the density of printed dots. It can achieve high spatial resolution and is free of Moire artifacts; but it may lack the required print stability needed for EP printing.; To combine the advantages of both AM and FM halftoning methods, we first developed an algorithm called AM/FM halftoning, which simultaneously modulates dot size and dot density to render gray levels. AM/FM halftoning first determines the position of each dot using a dispersed dot halftoning algorithm and a dot density curve. The size of each dot is then modulated according to a dot size curve. A measurement-based parameter design system is developed to optimize both the dot size and dot density curves. A specific implementation of AM/FM halftoning is developed for electro-photographic printers having subpixel modulation ability such as Hewlett-Packard's pulse width modulation (PWM) technology. Dot size diffusion (DSD) and dot size screening (DSS) techniques are also developed for use when the quantization levels of PWM are insufficient. For parallel hardware implementation of AM/FM halftoning, a boundary-stitching algorithm is developed to reduce the halftone texture artifacts across the stripe boundary. AM/FM halftoning is also developed for color printing.; To further improve the print stability and image quality, we generalize AM/FM halftoning to an algorithm called clustered AM/FM halftoning. Similar to AM/FM halftoning, it still simultaneously modulates dot size and dot density. Moreover, with tone-dependent optimal cluster control, the algorithm achieves even better print stability and image quality than can be achieved with AM/FM halftoning. Clustered AM/FM halftoning is also extended to color printing.