3 Oct. 08 - 3 jan. 09
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. 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.
(in Wikipedia )