Combining low-frequency and spread spectrum watermarking

Low-frequency watermarks and watermarks generated using spread spectrum techniques have complementary robustness properties. In this paper, we combine both watermarking paradigms to design an oblivious watermark that is capable of surviving an extremely wide range of severe image distortions. An image is first watermarked with a low-frequency pattern and then a spread spectrum signal is added to the watermarked image. Since both watermarks are embedded in a different portion of the frequency space, they do not interfere. For the low-frequency watermark, we modify the NEC scheme so that the original image is not needed for watermark extraction. The image is first normalized and the watermark is embedded into the lowest frequency discrete cosine modes by encoding a binary pattern (a watermark) using a special quantization-like function. The watermark detector uses a weighted correlation and a simple one-dimensional search to adjust for proper normalization. The low-frequency watermark is combined with a spread spectrum signal added to the middle frequencies of a DCT. The resulting double watermarked image is extremely robust with respect to a very wide range of quite severe image distortions including low-pass filtering, pixel permutations, JPEG compression, noise adding, and nonlinear deformations of the signal, such as gamma correction, histogram manipulation, and dithering.