Digital watermarking is a technique which allows to add hidden copyright or other verification messages to digital audio, video, or image signals and documents. Such hidden message is a group of bits describing information pertaining to the signal or to the creator of the signal (name, place, etc.). The technique takes its name from watermarking of paper or money as a security measure. Digital watermarking is a form of steganography, in which data is hidden in the message without the end user's knowledge.

While the addition of the hidden message to the signal does not restrict that signal's use, it provides a mechanism to track the signal to the original owner.

A watermark can be classified into two sub-types: visible and invisible. Visible watermarks change the signal altogether such that the watermarked signal is totally different from the actual signal, e.g., adding an image as a watermark to another image.

Invisible watermarks do not change the signal to a great extent, i.e., there are only minor variations in the output signal. An example of an invisible watermark is when some bits are added to an image modifying only its least significant bits.

There are various spatial and frequency domain techniques used for adding watermarks to and removing them from signals. Spatial techniques are not robust to some attacks to the signal like cropping and zooming, whereas most frequency domain techniques are quite robust to such attacks.

External links

• Knowledgebase.Aegisdrm.com (http://knowledgebase.aegisdrm.com/knowledgebase_digital_watermarking_drm.htm)— Directory of Books, Journals & Conferences on Digital Watermarking and Digital Watermarking Assessment Tools
• Digimarc.com (http://www.digimarc.com/)—Digital watermark software
• Watermarker.com (http://www.watermarker.com/)—Visible watermark software
• WatermarkFactory.com (http://www.watermarkfactory.com/)—Software that allows to add text and image watermarks to pictures of any format