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Counteracting geometric distortions for DCT based watermarking
| Details |
Inventors: Cox, Ingemar J.; Miller, Matthew L.;
Assignee: Signafy, Inc. (Princeton, NJ)
Primary Examiner: Boudreau; Leo H.
Assistant Examiner: Novik; Dmitry A.
Attorney, Agent or Firm: Feig; Philip J.
Detection of a watermark in blocks of video or image data which has been subjected to affine geometric distortion is accomplished by approximating the distortion by a spatially varying translation of the blocks. The watermark is extracted by spatial translations of the blocks based on an assumed affine geometric distortion. The extracted watermarks are correlated with possible watermarks and the maximum correlator output is tested for statistical significance in order to determine whether a watermark is present in the image or video frame. The data may be compressed or uncompressed data. If the affine geometric distortion is known, a search for the maximum correlator output over a space of possible distortions can be performed. |
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DETAILED DESCRIPTION A watermark algorithm divides an image or video data into 8.
times.
8 pixel blocks.
The blocks are then apportioned among 38 groups.
The watermark detection process begins by summing all 8.
times.
8 blocks in a group, thereby reducing the dimension of the data into 38 blocks of 8.
times.
8 values each.
A subset of coefficients in each of the 8.
times.
8 blocks is further summed together to form a one-dimensional vector of length 38.
The 38 groups of 8.
times.
8 blocks of DCT coefficients are chosen in such a way that translation shifts of multiples of 8 pixels simply result in a cyclic rotation of the extracted 1-D watermark vector.
For shifts that are not a multiple of 8, it is only necessary to determine the translation remainder modulo 8, i.
e.
translation of 0 to 7 pixels in both the x and y directions.
This is accomplished by taking the 38-8.
times.
8 blocks of DCT coefficients and first transforming them to the spatial domain.
Next, the data is shifted by one of the 64 possible translations before being transformed back to the DCT domain.
The watermark is then extracted and tested as will be described below.
The process is repeated for all translations and the maximum value of the correlator output is then tested for statistical significance in order to determine whether a watermark is present.
Since the amount of translation of the image or video frame is not known beforehand, by repeating the shift for all translations of 0 to 7 pixels, and then determining the signal with the highest correlation to a watermark, the most likely translation factor and watermark are determined.
The present invention approximates geometric distortions by spatially varying translations.
While removing the change or distortion from the image or video data is extremely difficult, manipulation in the form of spatial translation of blocks is a relatively simple way to extract a watermark from the distorted image or video, without restoring the image or video to its original, undistorted condition
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TV Signal 
