Collective blind image watermarking in DWT-DCT domain with adaptive embedding strength governed by quality metrics

In this paper the merits of block-based blind watermarking in a composite DWTDCT domain are explored. To improve the performance in robustness and imperceptibility, the quantization index modulation (QIM) applied to DWT-DCT coefficients has been formulated in an adaptive manner, where controlling parameters are designed to minimize the bit error rates of extracted watermarks subject to a quality criterion. The targeted coefficients are chosen based upon signal analysis in combination with additional consideration of human visual characteristics. To further enhance watermarking efficiency, two collective strategies are proposed. One takes advantage of multi-bit embedding and the other modifies the norm of a vector constituted by selected coefficients. Experimental results show that, in comparison with other DWT-and/or DCT-related watermarking methods, the proposed collective schemes achieve satisfactory improvements in robustness while the imperceptibility is properly maintained.