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Based on Radon transform and 2-D Fourier transform, this paper presents a novel digital image-watermarking scheme that is invariant to rotation, scale and translation (RST) attacks. By use of an invariant centroid as the origin of Radon transform, the watermark is embedded into a domain obtained by taking Radon transform of a circular area selected from the image, and then extracting 3-D Fourier magnitude of the Radon transformed image. Furthermore, to prevent the watermarked image from degrading due to inverse Radon transform, only watermark signal is inverse Radon transformed and then add to the original image. Experimental results demonstrate that the proposed scheme is robust to RST attacks.