Analysis of feature-based geometry invariant watermarking

Ensuring robustness against geometrical manipulations is a challenging task for watermarking algorithms. Watermarking algorithms that depend on the inherent geometry of the image for synchronization fail ungracefully due to de-synchronization through geometric manipulations. A class of algorithms tries to overcome this problem using feature-based synchronization markers. These are essentially composed of three building blocks: feature point extraction, elementary patch formation and registration to a standard geometry, and watermarking of elementary patches. Extracted feature points are bound to the image content, thus may be used as robust synchronization markers. This set of points is used to divide the image into elementary patches, which are warped into a standard geometry ensuring the exact synchronization during insertion and extraction. Once the synchronization is provided, any watermarking technique may reliably operate on elementary patches. In this paper, we present an analysis of feature-based geometry invariant watermarking algorithms. A discussion of the requirements on each building block is followed by potential solutions to meet these requirements. Furthermore, we present theoretical and practical limitations of these solutions via examples. In particular, segmentation based feature point extractors and triangulation based elementary patch formations are evaluated.