RST invariant digital image watermarking based on a new phase-only filtering method

摘要

Watermarked image may undergo geometric transforms which make watermark detection difficult or impossible. This paper presents a new rotation, scaling, and translation (RST) invariant digital image watermarking scheme based on log-polar mapping and a new phase-only filtering method. Watermark is embedded in the spatial domain imperceptibly based on the human perceptual model. Because of the log-polar mapping, the rotation and scaling in the spatial domain result in shifts in the log-polar mapping domain. Based on the new phase-only filtering method, the shift parameters in the log-polar mapping domain can be computed, and then the rotation degree and scaling ratio in the spatial domain can be successfully obtained through inverse log-polar mapping computation. The translation parameters in the spatial domain can also be calculated through the phase-only filtering method. After obtaining the RST parameters, the watermarked image can be transformed back to its original orientation, size, and position. Then the watermark can be detected easily and accurately. This scheme does not need an original image to detect the watermark. Only two templates derived from the original image are needed for the watermark detection. The size of each template is 64 x 64 bytes and can be compressed to 1 K bytes without loss of computation accuracy. Experiments show that the proposed scheme is robust against various attacks including geometric attacks. The method of inverting RST transforms can also be used in other watermarking algorithms.