KEYWORDS: Digital watermarking, Distortion, Data hiding, Interference (communication), Discrete wavelet transforms, Performance modeling, Systems modeling, Signal to noise ratio, Data modeling, Demodulation
Data hiding has been mainly studied in the last years. Many applications are
targeted such as copy-rights management, meta-data embedding for rich-media
applications, ... In all these applications, it is crucial to estimate what
is the capacity of data hiding. Many works have then been made to study
watermarking performance considering data-hiding as a kind of channel
communication. However in all these studies, an assumption is made about the
perfect knowledge of all attacks parameters (may be known in advance or later
estimated with attacks modeling). More especially a malicious attacker may
biased its attack so that parameters estimation may not be perfect
(desynchronization in parameters). Furthermore, random geometrical attacks
for images such as proposed by Stirmark benchmark (more generally
desynchronization attacks) show that perfect synchronization may not also be
achievable. These last kind of attacks are actually the most effective and
lack of theoretical modeling for capacity estimation. We then propose a new
model for taking into account desynchronization phenomenon in data hiding
(coupled with degrading attacks - i.e. optimal SAWGN attacks). Further,
thanks to the use of game theory, we state bounds on the capacity that may be
obtained by data hiding systems when subject to desynchronization.
In spread-spectrum watermarking, the watermarked document is obtained from the addition of an attenuated watermark signal to a
cover multimedia document. A traditional strategy consists of
optimizing the detector for a given embedding function. In
general, this leads to sub-optimal detection and much improvement
can be obtained by exploiting side-information available at the
embedder. In some prior art, the authors showed that for blind
detection of small signals, maximum detection power is obtained to
first order by setting the watermark signal to the gradient of the
detector. Recently, Malvar et al. improved the performance of direct-sequence spread-spectrum watermarking by using a signal dependent modulation. In the first part of the paper, we develop this idea further and extend Costa's decoding theory to the problem of watermarking detection. In the second part, we propose a practical implementation of this work using non-linear detectors based on our family of polynomial functions. We show some improved performance of the technique.
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