Digital watermarking provides a solution for protecting the copyright of digital contents, and print-and-scan resilient data hiding finds important applications in document security. In this paper, we propose a clustered dot halftone watermarking algorithm based on phase modulation. The watermark region and the other region use the threshold matrix for halftoning at the two different screen angle. The watermark is detected by overlaying the model, with one of the screen angle. Mean filtering and median filtering is performed on the recovered watermark to improve the appearance, and watermark is pre-processed using Arnold transformation to eliminate the edges in the border of the watermark. Experiments show watermark can appear under smearing, cropping and pint-and-scan attacks.

In order to resist print and scan attacks, a kind of digital halftone image watermarking algorithm is proposed, which is based on the edge detection and improved error diffusion. The edge of gray image is gotten using canny detection. Calculate the noise visibility function values of the edge points. Then, sort the values in ascending order and select the size that equal to the length of watermark as the location of watermark embedding. While the grayscale image turns to halftone image using the improved error diffusion algorithm, binary watermark is embedded in the edge position. Watermark is pretreated with Arnold before embedding to improve the safety of watermark. Experiment results show that the algorithm is good resistance to print and scan attacks, while resistance to shearing, noise and jpeg compression attacks.