Any serious stamp collector can tell you that watermarking has been around for a long time. These translucent designs in paper, visible when the paper is held to the light, are used to discourage forgeries. They can also indicate the paper’s manufacture. Computer-age watermarking applications are taking off. At least one computer printer manufacturer simulates a watermark pattern by printing lightly on the paper. The page text is then printed over the watermark. Now, photo banks, news agencies, libraries, and museums who own copyrights are interested in protecting digital images. There is also a need to protect digital video images and audio material.

The authors discuss uses and techniques of watermarking, as well as the issues associated with them. They group applications into three categories that reflect their uses: conveying ownership information; verifying that content has not changed; and providing information to a select group of recipients of the object. Though usually thought of as visible, watermarking can be invisible, with the information extracted by the computer.

There is a need for standards, especially if software decoding products are to be commercially successful. But, as the authors accurately observe, it is really too early for most watermarking standards, since the terminology, techniques, and applications are still emerging. The authors have developed tests that serve as a good starting point in the development of standard tests to determine if a watermark has been changed or removed. This is an excellent introduction to an important subject.