Nested object watermarking: transparency and capacity evaluation

Annotation watermarking (also called caption or illustration watermarking) is a specific application of image watermarking, where supplementary information is embedded directly in the media, linking it to media content, whereby it is not get separated from the media by non-malicious processing steps like image cropping or non-lossy compression. Nested object annotation watermarking (NOAWM) was recently introduced as a specialization within annotation watermarking, for embedding hierarchical object relations in photographic images. In earlier work, several techniques for NOAWM have been suggested and have shown some domain-specific problems with respect to transparency (i.e. preciseness of annotation regions) and robustness (i.e. synchronization problems due to high density, multiple watermarking), which is addressed in this paper. A first contribution of this paper is therefore proposed a theoretical framework to characterize requirements and properties of previous art and suggest a classification of known NOAWM schemes. The second aspect is the study of one specific transparency aspect, the preciseness of the spatial annotations preserved by NOAWM schemes, based on a new area-based quality measurement. Finally, the synchronization problems reported from earlier works is addressed. One possible solution is to use content-specific features of the image to support synchronization. We discuss various theoretical approaches based on for example visual hashes and image contouring and present experimental results.