Abstract: For embedding in the cover image are mostly binary watermarks, capacity is limited and needs to be improved anti-attack capability. In order to increase the capacity and enhance watermarking robustness of the algorithm, this paper proposes an embedded algorithm for gray watermarking image based on DWT and DCT. Firstly, the gray watermark image is preprocessed by Arnold scrambling. Secondly, after decomposing the selected cover image into the secondary layer, LL2, HL2, LH2 and HH2 of the secondary layer are selected to embed watermark. We apply 8×8 block DCT to each band and adopt an improved approach to embed high four bit planes of the gray watermark by switching two middle frequency coefficients. In order to enhance the robustness, the embedding intensity varies with different subbands. Finally, the simulation experiments show that the method is of strong robustness to cutting, image adding noise, histogram equalization, filtering, JPEG compression, and other attacks.

Key words: gray watermark image, discrete cosine transform, discrete wavelet transform, Arnold scrambling, bit planes, blind extraction