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In Lin's method [1], images are divided into non-overlapping three-pixel blocks. Each pixel block contains two pairs composed of two neighboring pixels. Absolute differences between pairs are calculated. And meanwhile, the absolute difference having the highest occurrence rate is obtained. 1-bit watermark is embedded into a pixel pair whose absolute difference is equal to this obtained value. Since each pixel block contains two differences, Lin's method achieves embedding rate of at most 2/3 bpp (bit per pixel) for a single embedding process. With the aim of further increasing embedding rate, we modify the embedding process in Lin's method to keep the third pixel of a pixel block unaltered in the proposed method. Then, this unchanged pixel is used again to reform into a new pixel block with its two neighboring pixels. As a result, the embedding rate can reach to 1 bpp for a single embedding process.

The paper presents a reversible watermark scheme based on hash function and lossless compression technology. The paper extracts the abstract information of the image as hash code and lossless compresses the LSB(Least Significant Bit) of the image to make some space where the image hash can be completely inserted. It needs to be done without introducing a large distortion to the image. The scheme can achieve the authentication purpose by hash code and restores the original image by retrieving the watermark sequence completely because of the lossless decompression of the LSB of the image.