Overview

Watermarking can be used in a wide variety of applications. In general, if it is useful to associate some additional information with a Work, this metadata can be embedded as a watermark. Of course, there are other ways to associate information with a Work, such as placing it in the header of a digital file, encoding it in a visible bar code on an image, or speaking it aloud as an introduction to an audio clip. The question arises: when is watermarking a better alternative? What can watermarking do that cannot be done with simpler techniques?

Watermarking is distinguished from other techniques in three important ways. First, watermarks are imperceptible. Unlike bar codes, they do not detract from the aesthetics of an image. Second, watermarks are inseparable from the Works in which they are embedded. Unlike header fields, they do not get removed when the Works are displayed or converted to other file formats. Finally, watermarks undergo the same transformations as the Works. This means that it is sometimes possible to learn something about those transformations by looking at the resulting watermarks. It is these three attributes that make watermarking invaluable for certain applications.

The performance of a given watermarking system can be evaluated on the basis of a small set of properties. For example, robustness describes how well watermarks survive common signal processing operations, fidelity describes how imperceptible the watermarks are, and so forth. The relative importance of these properties depends on the application for...