MASTER THESIS Video Watermarking - Computer Graphics Group ...

Despite the expectations, all the methods have gone pretty well.
Collusion by swapping macroblocks
Swapping macroblocks consists in picking out macroblocks from
participating copies producing one video sequence with differently watermarked
macroblocks. When obtaining hidden information bit from swapped macroblocks,
the pure watermark block does not correlate with the detected watermark block
and the hidden information bit value is not determined at all (the correlation sum
is 0) in the ideal case. In practice, the bit values oscillate uniformly around zero,
thus eliminate each other in the correlation sum. Only the rest of the right
macroblocks remains. Thereby, similar results to the cropping test (number of
copies should correspond to the cropping factor) are expected.
The simulation takes place in the embedding process as well. Required
number of watermarks is generated and swapped producing a single watermark.
This watermark is then embedded in an usual way.
The results are summarized in Table 13.
Table 13: Collusion by swapping macroblocks test results
Comparing the results with cropping test results, the cropping attack is
more successful. Nevertheless, the results are quite similar including the fact
that noise method is the least robust one if lower number of the copies participate
in the attack.
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