MASTER THESIS Video Watermarking - Computer Graphics Group ...

detection.
Conversion into 8×8 blocks is out of the question because of intra prediction.
The intra prediction process for a 4×4 sub-block within one 8×8 block uses the
other 4×4 sub-blocks within that 8×8 block. Therefore, when the 8×8 block is
watermarked, the error caused by watermark embedding should be compensated
in the 4×4 sub-blocks when using the other 4×4 sub-blocks for prediction. But the
compensation may severely impair the already embedded watermark...
There is only conversion into 4×4 blocks left. In this case, intra prediction
does not give trouble. The conversion is performed by decoding (i.e. dequantizing
and inverse transforming) a 8×8 block, partitioning into 4×4 blocks and encoding
(i.e. forward transforming and quantizing) the blocks in order to obtain transform
coefficients for watermark embedding. This works pretty well but the
quantization causes visible blocking artefacts.
With respect to the reasons above, no conversion is applied and only 4×4
blocks are taken for watermark embedding. 4×4 blocks have been chosen because
4×4 block transform is more usual than 8×8 transform. Video sequences given to
both embedding and detection can be converted to required format beforehand at
the cost of eventual quality degradation.
The pure watermark blocks are generated with dimension of 4 to cover 16
4×4 macroblock blocks.
Embedding into one block proceeds as follows. Only the half of transform
coefficients that represent higher frequencies are taken. Although higher
frequencies are more vulnerable to eventual attacks, human visual system is
more sensitive to distortion in lower frequencies and modification of low
frequency coefficients causes obtrusive blocking artefacts.
When 1 (the weight factor a in fact) is to be embedded, the coefficient with
the greatest absolute value is chosen. The coefficient modification keeps the sign
of the coefficient but eventually increases its absolute value to required
robustness level. If the coefficient is positive but lower than a, the coefficient is
increased to a. If the coefficient is negative but greater than -a, the coefficient is
decreased to -a. The coefficient is remained unchanged if it is greater than a in
absolute value. If the coefficient is 0, a or -a is randomly assigned. The purpose
why all this is done is to enforce a non-zero value in the half while producing to
the lowest distortion.
When -1 is to be embedded, all coefficients in the half are set to zero. This
causes loss in detail.
Detected watermark element values are obtained directly from transform
coefficients. If all transform coefficient values in the half are zero, -1 is returned,
otherwise (if there is at least one non-zero coefficient) 1 is returned.
Recompression at lower bit-rate causes more loss in detail, thus zero
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