Digital Watermarking

Digital watermarking is a pattern of bits inserted into digital content, such as a digital image, an audio or video file, that identifies the file's copyright information (name of the author, distribution rights, etc.). The name derives from the faint marks imprinted on paper stationery that identify the paper manufacturer. Watermarking is also known as data embedding and information hiding.

Digital watermarks provide copyright protection for digital intellectual property by "fingerprinting" it with a purchaser's identifying information. Illegal copies made and distributed can be traced back to the purchaser.

Digital watermarks are either visible or invisible. Visible watermarks change the content noticeably, usually by adding a copyright image in the background, as is done typically to Web sites and copyright documents. If the work is printed, the copyright image always displays as a part of the content.

Invisible watermarks do not change the content in an obvious manner. In one example, only the least significant bits of an image are modified. Invisible watermarking techniques have the steganographic feature of not being perceivable by the human eye.

To resist attacks, the actual bits representing a digital watermark must be scattered throughout the file so they cannot be identified and manipulated. Digital watermarks must be robust to withstand normal changes to the file, such as reductions from lossy compression algorithms like JPEG.

Techniques used for adding and removing digital watermarks include spatial and frequency domain. Purely spatial techniques do not fully protect against some attacks, like cropping and zooming; whereas most frequency-domain and mixed-domain techniques are robust to withstand such attacks.

Some of the most popular companies offering digital watermarking are Digimarc and Watermarking World.