CDMA-based watermarking schemes in DWT domain

CDMA-based watermarking is an effective technique since the watermark energy is dispersed over a large number of frequencies and the watermark energy embedded into each frequency is small. Due to the fact that, in the subspace spanned by the spreading codes, it is hard to find a projection vector which is approximated closely to the original cover image vector to significantly degrade the influence of cover image on the accuracy of watermark extraction. It motivates us to propose a variance-reduction technique to enhance the robustness of watermarking scheme in this paper.