Printer Models and Error Diffusion

A new model-based approach to digital halftoning is proposed. It is intended primarily for laser printers, which generate “distortions" such as “dot overlap.” Conventional methods, such as dustered-dot ordered dither, resist distortions at the expense of spatial and gray-scale resolution. The proposed approach relies on printer models that predict distortions, and rather than merely resisting them, it exploits them to increase, rather than decrease, both spatial and gray-scale resolution. We propose a general framework for printer models and find a specific model for laser printers. As an example of model-based halftoning, we propose a modification of error diffusion, which is often considered the best halftoning method for CRT displays with no significant distortions. The new version exploits the printer model to extend the benefits of error diffusion to printers. Experiments show that it provides high-quality reproductions with reasonable complexity. The new technique produces images that are sharper and have richer gray tones than those obtained with traditional techniques (e.g. “classical" screening). The quality of images printed on a 300-dpi printer using the new technique is comparable with that of images printed on a 400-dpi printer using traditional techniques. The proposed modified error diffusion technique is compared with Stucki's MECCA, which is a similar but not widely known technique that accounts for dot overlap. MECCA is more efficient computationally, whereas the proposed algorithm has better performance. Model-based halftoning can be especially useful in transmission of high-quality documents using high-fidelity gray-scale image encoders. As we show in a companion paper, in such cases, halftoning is performed at the receiver, just before printing. Apart from coding efficiency, this approach permits the halftoner to be tuned to the individual printer, whose characteristics may vary considerably from those of other printers, for example, write-black versus write-white laser printers.