MASTER THESIS Video Watermarking - Computer Graphics Group ...

At the beginning of the entire process, the same pure watermark as in the
embedding process is generated. The pure watermark is further used for
correlation with the detected watermark to retrieve hidden information.
In each macroblock, watermark block element values carrying one hidden
information bit are obtained using particular watermarking method (the scheme
is depicted in Figure 13). In case of frequency domain, the transform coefficients
are directly accessible. In case of spatial domain, the coefficients have to be
dequantized and inverse transformed in order to obtain the residual. It is further
added to predicted samples and clipped giving picture samples suitable for
detection. In this case, the residual is not enough because it depends on selected
intra prediction mode which could change after an attack.
Figure 13: Watermark element detection scheme
Obtained watermark block element values are compared with the
corresponding element values of the pure watermark. When the two
corresponding values match, 1 has been encoded, while when they differ, -1 has
been encoded. Remember the modulation of pure watermark by bits of hidden
information in the embedding process: when -1 was to be encoded, values of the
watermark block elements are inverted, i.e. each 1 becomes -1 and vice versa
(thus differ), and when 1 was to be encoded, values of the block elements remain
unchanged (thus match).
But in practice, especially after an attack, the corresponding values in one
block need not 100% match or differ. Therefore, some correlation mechanism has
to be proceeded. The correlation sum for i-th watermark block is computed:
where W P ij is j-th element value of the pure watermark block and W * ij is j-th
element value of the detected watermark block.
When the block encodes 1 (corresponding values match – they are either (1,
27
(8)