An Efficient Method to Improve the Quality of Watermarked Cover Image

In this paper, a general concept called n+k/n method is introduced. In n+k/n method, a special mapping function is defined to map an intermediate set with n+k bits into n bits information set. In the embedding process, the intermediate set is embedded into cover image rather than embedding the information set. If the distance between the features of cover image and the bits of intermediate set is less than that between the features and information bits, the better cover image quality can be gained. According to the idea of n+k/n method, a special case called 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 got. The one which has smaller distance from feature bits is selected as intermediate set. In the paper, it is proved that the maximun 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 n+1/n method will reach 25%, and the improving quality in embedded cover image is more than 2.5db..