Fast and Automatic Watermark Resynchronization based on Zernike Moments

摘要

In some applications such as real-time video applications, watermark detection needs to be performed in real time. To address image watermark robustness against geometric transformations such as the combination of rotation, scaling, translation and/or cropping (RST), many prior works choose exhaustive search method or template matching method to find the RST distortion parameters, then reverse the distortion to resynchronize the watermark. These methods typically impose huge computation burden because the search space is typically a multiple dimensional space. Some other prior works choose to embed watermarks in an RST invariant domain to meet the real time requirement. But it might be difficult to construct such an RST invariant domain. Zernike moments are useful tools in pattern recognition and image watermarking due to their orthogonality and rotation invariance property. In this paper, we propose a fast watermark resynchronization method based on Zernike moments, which requires only search over scaling factor to combat RST geometric distortion, thus significantly reducing the computation load. We apply the proposed method to circularly symmetric watermarking. According to Plancherel's Theorem and the rotation invariance property of Zernike moments, the rotation estimation only requires performing DFT on Zernike moments correlation value once. Thus for RST attack, we can estimate both rotation angle and scaling factor by searching for the scaling factor to find the overall maximum DFT magnitude mentioned above. With the estimated rotation angle and scaling factor parameters, the watermark can be resynchronized. In watermark detection, the normalized correlation between the watermark and the DFT magnitude of the test image is used. Our experimental results demonstrate the advantage of our proposed method. The watermarking scheme is robust to global RST distortion as well as JPEG compression. In particular, the watermark is robust to print-rescanning and randomization-bending local distortion in Stirmark 3.1.