Reducing prediction error in watermark retrieval process by multiple non-linear filtering

This paper presents an improving method used in the watermark retrieval process of digital watermarking for color image based on the modification of image pixels. Based on the prior knowledge of the embedded watermark pattern, some errors from the watermark bit prediction process can be reduced simply by changing the predicted bit value in accordance with all predicted bits value within the prediction area. To achieve this, we simply implement a non-linear filter with different characteristics to the predicted watermark bits, and output the result as the retrieved watermark. With the proposed method, the performance in both accuracy of the retrieved watermark and robustness of the embedded watermark can be improved. The experimental results show the improvements in term of NC at the equivalent PSNR after implementing our retrieval method with the existing watermarking scheme, compared to the previous methods. The improved robustness of the embedded watermark against common image processing based attacks and geometrical attacks are also evaluated and compared.