The technology for embedding digital watermarks in images is well known. Likewise, the technology for detecting and reading the data payload carried by digital watermarks is well known. Assignee's U.S. patent application Ser. No. 09/503,881, filed Feb. 14, 2000, and U.S. Pat. Nos. 5,862,260 and 6,122,403 illustrate examples of various watermarking techniques. Artisans in the field know even more. Commercial systems are available for performing such operations.
Many watermarking systems redundantly embed the same watermark data in multiple regions of an image. Often watermarking systems embed data in images in a perceptually adaptive manner. That is, the amount of watermark signal in each region of an image is adjusted in accordance with the characteristics of the image in the particular region. The watermark may even be absent in some regions of the image. The purpose of so adjusting the watermark signal is to insure that the watermark signal will not be visible to an ordinary viewer of the image. Since the strength of the watermark signal varies from region to region, the signal is more easily detected in some regions of an image than in other regions of the image.
Systems for detecting watermarks generally sequentially examine the various regions of an image, seeking to detect the watermark. Generally, the amount of computational resources available is limited and if a watermark is not detected in a region as a result of applying a certain amount of computational effort, the detection operation moves on to the next region of the image and the process is repeated.