MASTER THESIS Video Watermarking - Computer Graphics Group ...

that information, or each frame contains watermark with the same information,
then only a single frame should be enough.
In one frame, one single element of the watermark can be embedded into
one pixel, into a block of pixels or even into the whole frame.
2.2 Embedding and Detection
At first, general embedding and detection processes in raw uncompressed
images are described, then they are extended to compressed images.
Watermarking of a video sequence can be considered watermarking of a set of
single images but (especially in compressed video sequences) there are some
obstacles, as will be mentioned in Chapter 4.
Raw uncompressed images provide spatial domain by nature because values
of pixel color samples are directly accessible for modifications. For simplicity,
grey-scaled images are considered only.
Let us denote a picture to be watermarked by P and values of its pixel color
samples by Pi, a watermarked version of picture P by P * and values of its pixel
color samples by P * i. Let us have as many elements of watermark W with values
Wi as number of pixels in picture P. Watermark W hereby covers the whole
picture P. Further, it is possible to increase the watermark strength by
multiplying watermark element values by weight factor a. Then the natural
formula for embedding watermark W into picture P is:
That means that values of the watermark elements are simply added to values of
pixel color samples. But in practice, minimum and maximum values of the
samples have to be considered so the watermark can be impaired already during
the embedding process by clipping the results to the allowed range.
The detection process of the watermark is possible by computing inverse
function to (1) to derive possibly impaired watermark W*, therefore the original
picture P is needed.
In fingerprinting applications, watermark W* is then compared with the
original watermark W for statistical significance because it is more important to
check the presence of the watermark rather than fully retrieve hidden
information.
The requirement of the original picture for successful detection of the
watermark can be eliminated by using correlation (mentioned in Chapter 4), by
coding watermark element values into mutual relations among more pixels, or by
using different watermarking method.
For example, the following method could be used. Let us have a binary
10
(1)