In the broadest sense, watermarking is the incorporation of some data into content "essence" in such a way so that the data can subsequently be read (recovered) from copies of content. By essence we mean the data representing the picture and / or sound, and not some other side channel. The process of creating a watermark in a content item is called watermark embedding, and the additional data, in its embedded form, is called the watermark. The process of reading the watermark from a copy of the content is called watermark recovery.
Watermark technology is applicable to many media, but is commonly discussed in terms of motion video. This paper will take that perspective, although most of the concepts are directly translatable to sound or even to still images. Content will be consumed (viewed) using a variety of equipment types, commonly set-top boxes. But for the sake of generality, this paper refers to the content playback equipment as a "rendering device".
It is useful to conceptualize watermarking as a communications or signal processing problem. In this paradigm, the watermark information becomes the data to be communicated, and the content becomes the carrier signal. Indeed, the content carrying the watermark is often referred to as the host signal. From a communications standpoint, the perceptible features of the content constitute noise that interferes with the watermark's information signal. The watermark itself may consist of changes to the host signal features; the watermark may thus be viewed as modulating the host signal. In either case, the host signal constitutes a very noisy carrier for the watermark.
Common attributes of watermarks and watermark systems are:
Perceptible vs. Imperceptible Watermarks - Watermarks may either be apparent to the consumer when the content is rendered, or disguised in such a way that the viewer is unlikely to notice the watermark's presence. Perceptible watermarks are commonly used to designate ownership, exemplified by the quite visible logo appearing in many network broadcasts. Forensic watermarks, on the other hand, are typically designed to be imperceptible, in order to secure the watermark information and to avoid degrading the content. In general, watermarks fall along a continuum of perceptibility, according to the needs of the users and the capabilities of the technology.
The field of steganography, the technology of hiding messages in content such that the casual observer is unaware of the message's existence, includes imperceptible watermarking.
Readable vs. Detectable Watermarks - A watermark may carry only a single bit of information, that is, the watermark is significant only in its presence or absence. Such watermarks are classified as detectable. A readable watermark, by contrast, contains a more complex message, typically many bits of information. Mathematically, a readable watermark with N bits of information could be conceptualized as having been chosen from a set of 2N detectable watermarks. For a message of useful length, the number of marks in the set becomes unmanageable, so a practical readable watermark implementation must include a means of decomposing the watermark to reconstruct the message from independent parts.
Forensic watermarks must carry a message: a readable watermark, or a series of detectable watermarks is required. In the latter case, the message is treated as a series of independent bits, each of which is represented by a single detectable watermark.
