A mixed modulation scheme for blind image watermarking

Relative modulation (RM) is a scheme developed for effective blind image watermarking. The embedding of a binary bit can be implemented by first selecting a target coefficient along with its prediction and later manipulating this coefficient pair to maintain a desired inequality relationship. The RM has been confirmed to possess superior robustness against malign attacks but suffer undue image distortion. In this paper, a scheme is introduced to ameliorate the quality degradation problem due to excessive coefficient modulation in DCT-based image watermarking. By grafting quantization index modulation (QIM) onto RM, we enable the proposed mixed scheme to achieve a comparable peak signal-to-noise ratio and mean structural similarity and yet remain adequate robustness against commonly encountered attacks. Under the presupposition that the embedding strength is controlled at a similar level, the mixed modulation scheme can compete with the better one between the RM and QIM.