Digital watermarking is a technique which allows an individual to add hidden copyright notices or other verification messages to digital audio, video, or image signals and documents. Such a message is a group of bits describing information pertaining to the signal or to the author of the signal (name, place, etc.). The technique takes its name from watermarking of paper or money as a security measure. Digital watermarking can be a form of steganography, in which data is hidden in the message without the end user's knowledge.
A simple example of a digital watermark would be a visible "seal" placed over an image to identify the copyright. However the watermark might contain additional information including the identity of the purchaser of a particular copy of the material. According to the human perception, the digital watermarks can be divided into two different types as follows: visible and invisible. Visible watermark is a secondary translucent overlaid into the primary image as shown in the figure.
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. Stock photography agencies often add a watermark in the shape of a copyright symbol ("©") to previews of their images, so that the previews do not substitute for high-quality copies of the product included with a license.
Visible watermarks can be used in following cases :
* Visible watermarking for enhanced copyright protection.
In such situations, where images are made available through Internet and the content owner is concerned that the images will be used commercially (e.g. imprinting coffee mugs) without payment of royalties. Here the content owner desires an ownership mark, that is visually apparent, but which does not prevent image being used for other purposes (e.g. scholarly research).
* Visible watermarking used to indicate ownership originals.
In this case images are made available through the Internet and the content owner desires to indicate the ownership of the underlying materials (library manuscript), so an observer might be encouraged to patronize the institutions that owns the material.
Invisible watermarks do not change the signal to a perceptually 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. Invisible watermarks that are unknown to the end user are steganographic. 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.
Another application is to protect digital media by fingerprinting each copy with the purchaser's information. If the purchaser makes illegitimate copies, these will contain his name. Fingerprints are an extension to watermarking principle and can be both visible and invisible.
There are various spatial and frequency domain techniques used for adding watermarks to and removing them from signals. Purely spatial techniques are not robust to some attacks to the signal like cropping and zooming, whereas most frequency domain techniques and mixed-domain techniques are quite robust to such attacks.
The communication of a digital watermark may be viewed as an exercise in digital communication. The message bits are encoded and embedded in a suitable carrier. The properties that are desired of the watermark, such as imperceptibility, robustness to noise and to image editing such as cropping and rotation are the factors that drive the choice of carrier. In robust watermarks, it is the combination of low signal amplitude (because the watermark is invisible) and large bandwidth (because images are typically quite large), as well as the relatively short length of the message, that dictates the use of spread spectrum for encoding the message bits. Spread spectrum is a robust and secure form of communication. In image watermarking, the spread spectrum signal is typically placed in the frequency domain to produce a watermark that is immune to image processing.
Image compression techniques, such as JPEG, inspired the use of the frequency domain for embedding imperceptible watermarks in images. The first frequency domain technique was devised by Scott Burgett, Eckhard Koch, and Jian Zhao, who utilized the Discrete Cosine Transform. This and other transforms, such as the Wavelet transform, were used by Joseph O Ruanaidh, who later developed rotation and translation invariant watermarks based on the Fourier transform. Ingemar Cox popularized the use of Spread spectrum techniques for robust watermarking. Geoff Rhoads, Chief Technical Officer and founder of Digimarc Corporation, developed the PictureMarc technology.
