半色调图像的数字水印可广泛应用于打印／印刷输出的版权保护和证件、票据、货币的防伪，近几年来其研究取得了较大的进展。为使人们对该领域研究现状有一概略了解，首先论述了半色调图像水印的原理和主要研究方法；然后重点分析了直接嵌入法、间接嵌入法和半色调可视加密方法，并比较了它们的优缺点；同时通过比较发现：基于人眼视觉系统的间接嵌入法，在纸质作品的版权保护领域会有更广阔的应用前景；最后提出了鲁棒的半色调图像水印目前存在的问题和进一步的发展方向。

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.