Halftoning algorithms for robust digital printing

Digital halftoning provides a mechanism for rendering continuous-tone images on discrete devices that are capable of directly generating only a few different output levels. We present robust halftoning approaches for both monochrome and color printing. First we incorporate models for printer dot interactions for monochrome electro-photographic printers within the iterative Direct Binary Search (DBS) halftoning algorithm. Both analytical and stochastic models are studied. In the analytical model it is assumed that the printer can generate a circular spot with constant absorptance at each printer addressable location, whereas the stochastic model is based on microscopic absorptance and variance measurements. We also present an efficient strategy for evaluating the change in computational cost as the search progresses. With our scheme, updating the change in error only involves a few fetches from two look-up-tables and some scalar multiplications and additions. Experimental results are provided that show that DBS with an appropriate model for printer dot interactions yields enhanced detail rendition, suppression of aliasing artifacts, and improved tonal gradation in shadow areas. Next the problem of optimal periodic clustered dot (CD) color screen design is addressed. In CD color screening, the screens for each colorant are rotated to a different angle and offset relative to each other. If the angles and offsets are not carefully chosen, visible moiré and rosette artifacts appear. These artifacts primarily result from the interaction of the periodic structures associated with the halftone screens of different colorants. Registration errors also introduce un-wanted displacements in the individual screens. Using lattice theory and a model for the perceived rendered halftone, we present a systematic method for designing moiré and rosette free CD color screens that are robust to registration errors