MASTER THESIS Video Watermarking - Computer Graphics Group ...

file in order to provide detailed results for deeper analysis.
At the end of the entire detection process, average probability as average
value of absolute correlation coefficient values over all intra coded pictures is
estimated and printed. The probability expresses the detection success rate.
4.2.4 Notes
The generation of the watermark is based on spread spectrum technique
presented by Hartung [2]. Spread-spectrum watermarks are described in detail
in Section 4.3.1.
The watermark is embedded into luma samples only because human visual
system is more sensitive to changes in luminance than to chromatic components,
thus the watermark is harder to remove without severe quality degradation of
the cover content. Moreover, the chromatic channels of the video stream may be
completely removed and the video remains in former quality; the only difference
is that the video lacks colors.
To increase the watermark robustness, a different pseudo-random signal
can be generated for each picture in the embedding process. But then, a
synchronization mechanism must be implemented in the detection process to be
able to detect the watermark even if the order of the pictures is changed (or some
are missing or extra) by an attack. This is not trivial and is not implemented in
the plugin.
The generation of the pseudo-random signal is initialized by copy ID and
hidden information is created from content ID intentionally. If a cover content
copy contains more watermarks, they are represented by independent
pseudo-random signals, thus it is possible to detect each of them. If the pure
watermark were generated from content ID and hidden information from copy
ID, the advantage would be that the detection result would be just the copy ID
but copy IDs in multiple embedded watermarks would overwrite each other.
There is a weakness in the implementation because all macroblocks are
watermarked. Especially uniform areas in a picture are encoded by almost none
residual. Then, if such area is watermarked, the residual contains the watermark
alone, thus the watermark may be completely removed. The solution could be to
watermark only non-zero coefficients of residual with prejudice to robustness.
Another problem relates to watermarking of every macroblock. Because of
many coefficients that have been zero are altered to non-zero value, the bit-rate is
pretty much increased. We will see in Chapter 5 how high the increase is.
With respect to human visual system which is very sensitive to changes in
uniform areas, watermarking could be further improved to embed the watermark
only into edge features or textured areas. But the goal of the thesis is to compare
watermarking methods as they are, therefore no such extensions are
implemented.
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