Abstract：Inverse halftoning has been greatly developed recently and it is one of the key techniques in image processing for halftones. First the principles and implementations of inverse halftoning are given. Then the emphasis is put on the analysis of filtering methods

machine-learning based methods and optimum-estimation based methods for error diffusion halftones. Comparisons show that HVS-based fuzzy filtering schemes have a promising application. Finally the open issues and possible future research directions are pointed out.