1. Field of the Invention
The present invention relates generally to preventing unlawful copying of audio, video and other media that can be digitized and, more particularly, to a method for inserting a watermark into digitized data.
2. Prior Art
U.S. patent application Ser. No. 09/118,467, which is incorporated herein by its reference, describes a method for detecting watermarks in which a block DCT transform is applied to an image and then selected terms of the resulting block DCTs are summed to obtain a vector of values. This vector is then compared against an encoded watermark signal to determine whether that watermark has been inserted into the image. The corresponding watermark insertion algorithm, also described in U.S. patent application Ser. No. 09/118,467, proceeds by first summing the same block DCT terms to determine the vector already present in the image. This vector is random if the image has not been watermarked earlier. The inserter then chooses a new vector that resembles the original noise vector, but has a high correlation with the desired watermark signal. Next, it distributes the difference between the new vector and the old vector throughout the block DCT terms according to the relative sensitivity of the eye to changes in those terms. That is, large fractions of the difference are added to terms to which the eye is insensitive, while smaller fractions are added to the terms to which the eye is more sensitive. Finally, the inverse block DCT transform is taken to obtain a watermarked image.
The choice of using the block DCT transform in the preferred implementation of U.S. patent application Ser. No. 09/118,467 was made for several reasons. The primary reason is that the DCT is the transform used in MPEG video compression and JPEG still image compression. Thus, employing it for watermarking allows rapid watermark detection compressed data. A secondary, but more fundamental, reason is that block DCTs are a fast way to separate low frequency data (which is robust to most image transformations) from high frequencies (which is more fragile).
However, block DCTs, or any block-based transforms, are not a good representation for the perceptual modeling required during insertion. Perceptual models based on block transforms tend to ignore the interaction between noise added to neighboring blocks, resulting in a type of "blocking artifact," such as that visible in highly compressed MPEG and JPEG images. These same artifacts are also visible in images that have been too strongly watermarked by the insertion method of U.S. patent application Ser. No. 09/118,467.
The present invention improves on the insertion method of U.S. patent application Ser. No. 09/118,467 by allowing the image noise to be distributed according to a perceptual model based on more appropriate image representations, such as pixels or wavelets, while still generating a watermark detectable by a DCT-based detection method. It, thus, achieves higher image fidelity than the earlier insertion algorithm, while maintaining the advantages of block DCTs for speed, frequency separation and relationship to compression algorithms.