US Patent 7457002 Reverse diffusion digital halftone quantization

An improved digital halftoning method that uses an input image's global gray levels to determine the local gray levels of a monochrome output image. Input multi-bit pixels grouped into two-pixel-by-two-pixel local subcells are variously aggregated into one or more larger supercells. The size of said supercell(s) is related to and limited by the size of the global input bitmap. A final monochrome gray level is derived from said supercell(s) and distributed within contained subcells. Subcell gray levels are expressed as interim whole monochrome pixels and gray level remainders. A comparison is made of the final supercell and the summed interim subcell monochrome gray levels. An ordering of the remainders is used for assignment of additional monochrome pixels, if necessary, to yield final subcell monochrome gray levels. Gray level rounding errors thus are quantized by reverse diffusion until a monochrome gray level for each of the global image's two-pixel-by-two-pixel local subcells is derived.