An efficient method to improve the quality of watermarked cover image

In this paper, a general concept called the n+k/n method is introduced. In the n+k/n method, a special mapping function is defined to map an intermediate set with n+k bits into a n bit information set. In the embedding process, the intermediate set is embedded into a cover image rather than embedding the information set. If the distance between features of the cover image and the bits of the intermediate set is less than that between the features and information bits, the better the cover image quality. According to the idea of the n+k/n method, a special case called the n+1/n method is proposed. In this method, the mapping function is an exclusive-or operation. When an information set is given, two sets which satisfy the mapping function can be obtained. That which has a smaller distance from feature bits is selected as an intermediate set. In the paper, it is proved that the maximum distance between the intermediate set and feature set is less than the average distance between the information set and the feature set. The reduction rate of feature modification for the n+1/n method will reach 25%, and the improving quality in the embedded cover image is more than 2.5 db.