Ranked dither for robust color printing

A spatially-adaptive method for color printing is proposed that is robust to printer instabilities, reproduces smooth regions with the quality of ordered dither, reproduces sharp edges significantly better than ordered dither, and may be less susceptible to moire. The new method acts in parallel on square, non-overlapping blocks of each color plane of the image. For blocks with low spatial activity, standard ordered dither is used, which ensures that smooth regions are printed with acceptable quality. Blocks with high spatial activity are halftoned with a proposed variant of dither, called ranked dither. Ranked dither uses the the same ordered dither matrix as standard dither, but the ranks of the thresholds are used rather than the thresholds themselves. Ranked dither is more sensitive than ordered dither to edges and more accurately reproduces sharp edges. Experiments were done with standard ordered dither masks of size 130, 130, 128, 144 for the cyan, magenta, yellow, and black planes respectively. Both on-screen and in-print, the results were sharper halftones. The entire process can be implemented in parallel and is not computationally expensive.