Robust Image Watermark Using Radon Transform and Bispectrum Invariants

Image watermark that is resistant to geometric distortion is remained to be an unsolved problem. Difficulty of the probelm comes from the situation that the watermark should be extracted without any information of the original image. In this paper, we review this problem and propose a new watermarking scheme based on invariant pattern recognition theory. We propose an invariant watermark using the Radon transform and higher order spectra. A bispectrum feature vector of the image is used as the watermark. Our approach differs from the previous methods in that we embed watermark into the phase of the higher order spectra. Also, our Radon embedding grid outperforms the Fourier-Mellin based methods. We devised a new embedding method which allows detection of the watermark when there is no exact inverse function during embedding. As we use the Radon transform, our method can be used for medical images. We show the invariance of the designed watermark with mathematical proofs. Experimental results confirm that this scheme is resistant to geometric distortions.