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A ROBUST IMAGE WATERMARKING SCHEME BASED ON THE ALPHA-BETA SPACE



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A ROBUST IMAGE WATERMARKING SCHEME BASED ON THE ALPHA BETA SPACE P Martins and P Carvalho Centre for Informatics and Systems University of Coimbra Po lo II Pinhal de Marrocos 3030 290 Coimbra Portugal pjmm carvalho dei uc pt ABSTRACT A robust image watermarking scheme relying on an af ne invariant embedding domain is presented The invariant space is obtained by triangulating the image using af ne invariant interest points as vertices and performing an invariant triangle representation with respect to af ne transformations based on the barycentric coordinates system The watermark is encoded via quantization index modulation with an adaptive quantization step 1 INTRODUCTION The robustness with respect to geometric transformations has been a widely studied topic in digital image watermarking leading to the introduction of several solutions trying to overcome the effects carried out by these distortions specially the de synchronization effect since most of the watermarking schemes perform a detection based on a correlation measure thus if a marked image is geometrically distorted the detector will correlate wrong image components 1 Robust schemes can be be grouped into four categories invariant domain based methods template based methods self synchronization based methods and content based methods The rst category includes methods which exploit invariant or partially invariant domains for watermark insertion e g applying the Fourier Mellin transform 2 or the Radon transform 3 The main drawback of invariant based methods is that they usually require interpolation in order to obtain the invariant domain at least two interpolation stages are present in an invariantbased scheme one to obtain the invariant space and embed the watermark into it and another one to perform the inverse mapping Because of the inaccuracy of interpolation methods the performance of these solutions is highly affected Methods from the second category provide robustness to geometric distortions by



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