Watermark embedding: Hiding a signal within a cover image

When looked at as a communication task, the watermarking process can be spl it into three main steps: watermark generation and embedding (information t ransmission), possible attacks (transmission through the channel), and wate rmark retrieval (information decoding at the receiver side). In this articl e we review the main issues in watermark generation and embedding. By focus ing on the case of image watermarking, we first discuss the choice of the i mage features the watermark is superimposed to. Then we consider watermark generation and the rule used to insert the watermark within the host featur es. By adopting again a communication perspective, some useful hints are gi ven on the way the watermark should be shaped and inserted within the host document for increased robustness against attacks. Given that invisibility is one of the main requirements a watermark must satisfy, the way psycho-vi sual notions can be used to effectively hide the watermark within an image is carefully reviewed in the second part of the article. Rather than insist ing on the mathematical aspects of each of the above issues, the main ratio nale behind the most commonly adopted approaches is given, as well as some illustrative examples.