Fragile and robust watermarking by histogram specification

This paper presents new results on regional image watermarking by exact histogram specification. Image is split in regions and for each region a watermark is specified. Watermarks are selected such as image original histogram is preserved. Main improvement of proposed regional scheme consists in the marking of the entire image (all the regions) with complementary watermarks. This procedure considerably increases watermarking robustness. The region selection strategy is discussed so that direct identification of regions and bordering effects are eliminated. Robustness/fragility of the proposed scheme depends on the specified histograms. In a general setting, exact histogram specification allows only certain graylevel values for the pixels of each region. Fragile watermarking is obtained when sentinel pixels' region is allowed to take only certain discrete values. Thus, using sparse histograms, one achieves not only image authentication, but also, in case of any attack or malicious editing, the detection of the area where image has been altered. On the contrary, robust watermarking against many attacks is obtained when pixels of each region are allowed to take values on compact intervals of graylevels.