A Color Opponency Watermarking Scheme for Digital Images

In this paper we present a novel watermarking scheme to generalize a previous proposal by the same authors [2]. In that paper to watermark the image amounts to process the colors in the picture as points in the Color Opponency space and to offset each one of them by a random vector. As an extra constraint, in order to avoid picture quality degradation, the offset applied should be such that a color is not moved outside of a small sphere. The watermarking algorithm processes the colors in the picture as points in the Color Opponency space and offsets each one of them, inside a suitable imperceptive area, by a random vector. To improve robustness we suggest to partition the whole pixels population into "color sets": each set gathers pixels with the same color. Each color set, exceeding a given cardinality, is in turn randomly partitioned into three subsets. Each one of these subsets is hence manipulated in different suitable ways. The watermark is obtained maintaining a record of the statistical distribution of the three subsets. Rigorous theoretical statistical analysis shows that this approach is robust with respect to the most common operations of image processing.