Digital watermarking
From ArticleWorld
Digital watermaking is a technique to add hidden messages to digital image, video or audio materials. The technique is similar, as a result, with the process of watermaking paper money as a measure of security.
It should be noted that digital watermaking is not a form of steganography. Its purpose is not to send message hidden in an image, but rather to track the copyright owner without affecting the material itself. Some watermaking techniques do go as far as introducing a hardly visible notice into the signal, or even making it completely invisible.
The term "digital watermark" was coined in 1992, in a paper by Andrew Tirkel and Charles Osbourne, in a paper called 'Electronic Water Mark' (DICTA 93, Macquarie University. p.666-673). However, similar techniques were already used at the time.
Visible and invisible
Watermarks generally fall in two categories:
- Visible watermarks, which change the signal (audio, video or static image). It is common for websites that provide high-quality photos to provide lower-resolution samples with a large copyright symbol applied, so that the preview does not substitute the final image. In this case, watermarks are also used as a marketing tool. Visible watermarks are also used by software developers to protect their applications: unregistered copies of a program may apply a large watermark on top of printed documents.
- Invisible watermarks, which do change the signal but not enough to be perceived. This is usually done by modifying the least significant bits in the packets of a signal. Some spatial and frequency domain exist and are commonly used to add and remove watermarks to and from signals. However, purely spatial techniques are not as efficient as mixed-domains techniques. It is common practice to use frequency domain techniques when mixed-domain techniques are not available.
Most general-purpose drawing and photo manipulation tools today have the option of applying watermarks on images.