Image analysis – Applications
Reexamination Certificate
1998-09-02
2001-06-26
Johns, Andrew W. (Department: 2621)
Image analysis
Applications
Reexamination Certificate
active
06252972
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a method and arrangement for detecting a watermark embedded in an information signal.
BACKGROUND OF THE INVENTION
Watermarks are perceptually invisible messages embedded in information signals such as multimedia material, e.g. audio, still pictures, animations or video. Watermarks can be used to identify the copyright ownership of information. They allow a copyright owner to trace illegal copies of his material by inspecting whether his watermark is present in said copies.
Watermarks are embedded in an information signal by modifying data samples of the signal (e.g. audio samples of an audio signal, pixels of an image, transform coefficients of a transform-coded signal, etc.) such that the original is not perceptibly affected. Various methods of watermarking are known in the art. For example, pixels of an original image are slightly incremented or decremented in accordance with corresponding bits of a binary watermark pattern.
In order to detect whether an information signal has an embedded watermark, the signal is subjected to a statistical analysis. The statistical analysis yields a parameter, hereinafter referred to as “decision variable”, which indicates to which extent the watermark is present in the signal. For example, if an image signal is watermarked by incrementing or decrementing its pixels in accordance with a watermark pattern, the decision variable may be the amount of correlation between the signal and an applied reference copy of the watermark. If an image is watermarked by modifying selected pixels, a prediction for said pixels is calculated from temporally or spatially adjacent pixels. The decision variable may then be the number of pixels being sufficiently different from their prediction.
Watermark detectors generate a binary output signal indicating “watermark found” or “no watermark found”. That is achieved by comparing the decision variable with a predetermined threshold. If the value of the decision variable exceeds the threshold, the watermark is considered to be present in the signal. The threshold value is decisive for the performance of a watermark detector. If the threshold is too low, the detector will often make a false positive decision (false alarm). If the threshold is too high, the detector will often make a false negative decision (missed detection). In particular, pixel-domain watermarks are vulnerable to false decisions. Copy protection by means of watermark detection can only be applied in consumer products if the probabilities of false alarms is sufficiently small.
OBJECT AND SUMMARY OF THE INVENTION
It is an object of the invention to provide an improved method and arrangement for detecting a watermark.
To that end, the method in accordance with the invention is characterized in that it comprises means for generating the threshold value in dependence upon statistical characteristics of the information signal and a desired probability of erroneous detections. Herewith is achieved that the threshold value is adaptively controlled to obtain a desired false alarm rate.
Further advantageous embodiments of the invention are defined in the dependent claims.
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patent: 6131161 (2000-10-01), Linnartz
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Boney et al., “Digital Watermarks for Audio Signals,” .Proc. 3rd IEEE int. Conf. on Multimedia Computing and Systems,Jun. 17-23 1996, pp. 473-480.*
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Information Hiding Second International Workshop, IH'98, Apr. 1998, (Portland, Oregon, USA), Jean-Paul M.G. Linnartz et al., Springer-Verlag Berlin Heidelberg 1998, “Analysis of the Sensitivity Attack Against Electronic Watermarks in Images”, p. 258-272.
Belk Michael E.
Johns Andrew W.
U.S. Philips Corporation
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