Digital watermark for printed materials

With the emergence of high resolution scanning and printing devices, copyright protection for printed materials becomes an important problem in digital image research. After analyzing geometric and pixel value distortion of an image scanned from legal and pirated printed materials respectively, we propose in this paper a copyright protection method based on digital image watermarking. Our algorithm first embeds watermark information into the geometric invariant Fourier-Mellin domain of the image to weaken the geometric distortion brought by legal print-and-scan process. The algorithm then rectifies image scanned from printed material using Radon transform and detected the watermark from it. Because there is an obvious difference between destruction degrees on the watermark caused respectively by legal and pirated printing and scanning process, the algorithm can computes the ratio of the correctly extracted watermark bits and compared it with a predefined threshold to authenticate the printed materials. Experiments show that our semi-fragile watermark algorithm can overcome some shortcoming of the existed algorithms and can be applied in copyright protection of varieties of printed materials.