Review of watermarking and the importance of perceptual modeling

A watermark embeds an imperceptible signal into data such as audio, video and images, for a variety of purposes, including captioning and copyright control. In this paper, we first outline the desirable characteristics of digital watermarks. Previous work in digital watermarking is then reviewed. Early work identified redundant properties of an image (or its encoding) that can be modified to encode watermarking information. The early emphasis was on hiding data, since the envisioned applications were not concerned with signal distortions or intentional tampering that might remove a watermark. However, as watermarks are increasingly used for purposes of copyright control, robustness to common signal transformations and resistance to tampering have become important considerations. Researchers have recently recognized the importance of perceptual modeling and the need to embed a signal in perceptually significant regions of an image, especially if the watermark is to survive lossy compression. However, this requirement conflicts with the need for the watermark to be imperceptible. Several recent approaches that address these issues are discussed.