Discrete wavelet transform based multiple watermarking scheme

The low-frequency embedding of the watermark increases the robustness with respect to image distortions that have low pass characteristics like filtering, lossy compression, geometrical distortions. On the other hand, oblivious schemes with low-frequency watermarks are more sensitive to modifications of the histogram, such as contrast/brightness adjustment, gamma correction, histogram equalization, and cropping. Watermarks inserted into middle and high frequencies are typically less robust to low-pass filtering lossy compression and small geometric deformations of the image, but are extremely robust with respect to noise adding, nonlinear deformations of the gray scale. It is understandable that the advantages and disadvantages of low and middle-to-high frequency watermarks are complementary. It appears that by embedding both watermarks into one image, one could achieve extremely high robustness properties with respect to a large spectrum of image processing operations. The above reasoning leads to proposed technique of embedding multiple watermarks into the low frequency and high frequency bands of discrete wavelet transform.