Watermarking techniques for halftone images have been well developed recently. They are widely used for protecting the copyright of printouts and preventing card or ticket

as well as currency from illegal forgery. Firstly the principle and implementation of watermarking for halftone images are given. Then the analysis of direct-embedding methods

indirect-embedding methods and halftone visual cryptography methods is conducted. The comparisons show that the HVS-based indirect-embedding schemes have a promising application in the copyright control of paper products. Finally the open issues and possible future research directions are pointed out.