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Based on log-polar mapping (LPM) and phase correlation, the paper presents a novel digital image watermarking scheme that is invariant to rotation, scaling, and translation (RST). We embed a watermark in the LPMs of the Fourier magnitude spectrum of an original image, and use the phase correlation between the LPM of the original image and the LPM of the watermarked image to calculate the displacement of watermark positions in the LPM domain. The scheme preserves the image quality by avoiding computing the inverse log-polar mapping (ILPM), and produces smaller correlation coefficients for unwatermarked images by using phase correlation to avoid exhaustive search. The evaluations demonstrate that the scheme is invariant to rotation and translation, invariant to scaling when the scale is in a reasonable range, and very robust to JPEG compression.