Characterisation of geometric distortions attacks in robust watermarking

Robust image watermarking algorithms have been proposed among others as methods for discouraging illicit copying and distribution of copyright material. Having robustness to pixels modifications in mind, many watermarking designers use techniques coming from the communications domain such as spreading spectrum to embed hidden information, be it in the spatial or in the transform domain. Most of the attacks dedicated to make watermarking algorithms inefficient degrade images through geometric distortions. One solution to counter them is to add synchronization information. In this paper we present an analysis of this type of distortions and we propose a metric to estimate the distortion undergone by an image. This metric is content independent, invariant to global translation, rotation and scaling, which can be considered as non-meaningful transformations. To demonstrate the relevance of this metric, we compare some of its results with the subjective degradation of the image produced by the Stirmark software