Rotation invariant feature extraction for watermarking

Many watermarks for still images are robust against common signal processing techniques, mainly JPEG compression, noise adding and low-pass filtering, while they are sensitive to geometrical manipulations, that yield desynchronization errors. In this paper robustness against some geometric transformations is achieved using a feature extraction method based on the Radon transform and whose aim is to identify an unique (and robust) feature from the image spectrum. The embedding, which exploits the extracted feature, is based on a multiplicative rule technique and is applied on a suitable subset of the image Fourier transform. The properties of the extracted feature allows to resynchronize the detector and the embedded watermark even if the image undergoes geometric manipulations (in particular rotations) as well as other processings, so that the correct watermark retrieving is guaranteed. Experimental results, lead on many standard images, confirm the effectiveness of the feature extraction method and the robustness of the watermark against both processing and geometric transformations.