Digital watermarking is the process of embedding information into a digital signal. The signal may be audio, pictures or video, for example. If the signal is copied, then the information is also carried in the copy.

In visible watermarking, the information is visible in the picture or video. Typically, the information is text or a logo which identifies the owner of the media. The image on the right has a visible watermark. When a television broadcaster adds its logo to the corner of transmitted video, this is also a visible watermark.

In invisible watermarking, information is added as digital data to audio, picture or video, but it cannot be perceived as such. An important application of invisible watermarking is to copyright protection systems, which are intended to prevent or deter unauthorized copying of digital media. Steganography is an application of digital watermarking, where two parties communicate a secret message embedded in the digital signal. Annotation of digital photographs with descriptive information is another application of invisible watermarking. While some file formats for digital media can contain additional information called metadata, digital watermarking is distinct in that the data is carried in the signal itself.

The use of the word of watermarking is derived from the much older notion of placing a visible watermark on paper.

The information to be embedded is called a digital watermark, although in some contexts the phrase digital watermark means the difference between the watermarked signal and the cover signal. The signal where the watermark is to be embedded is called the host signal.

A watermarking system is usually divided into three distinct steps, embedding, attack and detection. In embedding, an algorithm accepts the host and the data to be embedded and produces a watermarked signal.

The watermarked signal is then transmitted or stored, usually transmitted to another person. If this person makes a modification, this is called an attack. While the modification may not be malicious, the term attack arises from copyright protection application, where pirates attempt to remove the digital watermark through modification. There are many possible modifications, for example, lossy compression of the data, cropping an image or video, or intentionally adding noise.

Detection (often called extraction) is an algorithm which is applied to the attacked signal to attempt to extract the watermark from it. If the signal was unmodified during transmission, then the watermark is still present and it can be extracted. In robust watermarking applications, the extraction algorithm should be able to correctly produce the watermark, even if the modifications were strong. In fragile watermarking, the extraction algorithm should fail if any change is made to the signal.

In general, the fundamental watermarking parameters are classifies into 7 watermarking properties (alphabetic order):

- Capacity
- Complexity
- Invertibility
- Robustness
- Security
- Transparency
- Verification

A digital watermark is called robust with respect to a class of transformations T if the embedded information can reliably be detected from the marked signal even if degraded by any transformation in T. Typical image degradations are JPEG compression, rotation, cropping, additive noise and quantization. For video content temporal modifications and MPEG compression are often added to this list. A watermark is called imperceptible if the cover signal and marked signal are indistinguishable with respect to an appropriate perceptual metric. In general it is easy to create robust watermarks or imperceptible watermarks, but the creation of robust and imperceptible watermarks has proven to be quite challenging. Robust imperceptible watermarks have been proposed as tool for the protection of digital content, for example as an embedded 'no-copy-allowed' flag in professional video content. Digital watermarking techniques can be classified in several ways:

- Robustness
* A watermark is called fragile if it fails to be detected after the slightest modification. Fragile watermarks are commonly used for tamper detection (integrity proof).
* A watermark is called semi-fragile if it resist benign transformations but fails detection after malignant transformations. Semi-fragile watermarks are commonly used to detect malignant transformations.
* A watermark is called robust if it resists a designated class of transformations. Robust watermarks are commonly used in copyright applications (to carry ownership or forensic information) and copy protection applications (to carry copy and access control information).

- Perceptibility
* A watermark is called imperceptible if the original cover signal and the marked signal are (close to) perceptually indistinguishable.
* A watermark is called perceptible if its presence in the marked signal is noticeable, but non-intrusive.
* Modification to an original work that are clearly noticeable are commonly not referred to as watermarks, but referred to as generalized barcodes.

- Capacity
The length of the embedded message | m | determines two different main classes of watermarking schemes:
-
* | m | = 0: The message m is conceptually zero-bit long and the system is designed in order to detect Italic only the presence or the absence of the watermark w in the marked object SE. This kind of watermarking schemes is usually referred to as Italic zero-bit or Italic presence watermarking schemes. Sometimes, this type of watermarking scheme is called 1-bit watermark, because a 1 denotes the presence and a 0the absence of a watermark.
* | m | = n > 0: The message m is a n-bit long stream (m=m_1\ldots m_n,\; n\in\N, with n = | m | ) or M = {0,1}n and is modulated in w. This kind of schemes is usually referred to as multiple bit watermarking or non zero-bit watermarking schemes.

- Embedding method
* A watermarking method is referred to as spread-spectrum if the marked signal is obtained by an additive modification. Spread-spectrum watermarks are known to be modestly robust, but also to have a low information capacity due to host interference.
* A watermarking method is referred to be of quantization type if the marked signal is obtained by quantization. Quantization watermarks suffer from low robustness, but have a high information capacity due to rejection of host interference.
* A watermarking method is referred to as amplitude modulation if the marked signal is embedded by additive modification method which it similar to spread spectrum method but this method is especially embedded in spatial domain.

Digital Watermarking can be used for a wide range of applications such as:

- Copyright protection.
- Fingerprinting (Different recipients get differently watermarked content).
- Broadcast Monitoring (Television news often contains watermarked video from international agencies).
- Covert Communication (steganography).

The evaluation of digital watermarking schemes can provide detailed information for watermark designer or end users. Therefore, different evaluation strategies exists.

Often used from watermark designer is the evaluation of single properties to show, for example, an improvement. End users, are mostly not interested in detailed information. They want to know, if a given digital watermarking algorithm can be used for their application scenario, and if yes, which parameter sets seems to be the best.