Robust oblivious digital watermarking using image transform phase modulation

Digital watermarking is the process of secretly embedding a short sequence of information containing owner identity or copyright information inside a digital signal, without changing its perceptual quality. The information is embedded in a way that is difficult to erase. We present a new oblivious digital watermarking method for copyright protection of still images. The technique is based on modifying the sign of a subset of low frequency, image transform coefficients with high to moderate magnitudes. These transforms include the DCT, DFT and Hadamard transforms. Watermark detection is done in an oblivious way. The robustness to a number of standard image processing attacks is demonstrated using the criteria of the latest StirMark benchmark test. The technique can be used also for other applications such as image fingerprinting.