“ Multiple Channel Watermarking of Color Images ”

A novel scheme for the watermarking of color images is presented in this communication. An attempt has been made to find the most appropriate alternative to the RGB color space. Color spaces with linear relation to the RGB color space with uncorrelated components are found to be most suitable for watermarking. Another objective has been to propose an adaptive watermarking scheme for any type of color image, including computer generated color images. The adaptivity is achieved by keeping the visibility metric (PSNR) in a predefined quality range while changing the watermark embedding strength. The length of the watermark is kept constant for each of the color channels. In order to select the suitable space for watermark casting each of the color channels is transformed through Discrete Wavelet Transform and then the Discrete Hadamard Transform is applied to the approximations only. By this multiple transformation we are in a position to select the most appropriate coefficients for the watermarking, which are close to the low frequencies and uncorrelated in nature. Higher level of security is achieved by introducing encryption among the selected transformed coefficients using a private encryption key. Detection is oblivious and only the watermark generation and the coefficient encryption keys are required for the detection, and not the original image. Scheme is tested against a variety of legitimate and illicit image processing operations including compression, scaling, uniform or gaussian noise addition, median filtering, cropping, multiple watermarking and StirMark benchmark. ACADEMIC RESEARCH COMPUTER TECHNOLOGY INSTITUTE 2002 ________________________________________________________________________________ _______________________________________________________________________________________ TECHNICAL REPORT No. TR2002/02/03 2