The technical challenge that watermarking presents is the successful concealment of the watermark signal in a much larger bandwidth medium. As [AP98] points out, usually we are more concerned with an attacker's ability to read or, worse, change or remove the watermark, than their ability to detect its presence. In the face of perfect compression methods, it is not clear that it is possible to conceal the presence of a watermark. For our watermark to be robust, the watermark information must be embedded in the target medium in such a way that removing this information irreparably damages the medium.
When considering static images, the commonly recognized transformations that a watermark should survive are:
All of these transformations in some sense preserve the value of the image to the user. After they have been applied, the image is still recognizably derived from the original image. This concept can be used as the foundation of a robust watermarking method [RP97]; if the image can be transformed into some space which is invariant to the value-preserving transforms listed above, the watermark can be applied in that space before reversing the transformation to arrive back at the original image. This watermark will then also be invariant under these transformations.
An often-used method for covert channel communication over a higher-bandwidth medium is spread spectrum communications. This method can also be applied to watermarking images, as described in [CKLS96,ORDB96]. It has the advantage of having a long history in both military and civilian digital communications, and is thus both well understood and of proven reliability. At the current time, it appears the most promising framework for embedding watermarks in either images or other digital media data streams.
Watermarks are most often used for copyright control. It is commonly recognized that no watermarking method can withstand all possible attacks. Thus, in the domain of network-accessible digital media, it may be better to regard a watermark as an aid to quickly comparing possible copies of a copyrighted work to the original, in order to locate copyright violations, than as an indicator of copyright in itself.