AP_06_4.vp

1 About watermarking
Watermarking is a type of steganography, with the help of

which we can embed information into the host medium, in
our case into a video stream. Its main task is to protect the
medium. To embed the information we use a key (stego key),
and the same key is used during the watermark detection
process [1].

The watermarking process consists of the following steps
(Fig. 1.):

Creation of the watermark (W): coding the data to embed
(D) into a format that is acceptable as input for the embed-
ding algorithm.

Using the stego key (S), we embed the watermark into the
host medium (M). The medium, which contains the water-
mark is called a stego medium (Mw).

The medium passes through a transmission channel,
where it can suffer various distortions (intended or unin-
tended). We will refer to this as a distorted medium (Mw’).

Using the stego key again, we search the hidden water-
mark in the medium. The watermark has probably suffered
distortions too, and we have to watch for this during retrieval.
Some algorithms use the original medium, but this is not
necessary. If we detect the watermark without the original
medium, we speak about blind watermarking.

We decode the retrieved watermark, and get the embed-
ded data.

2 Requirements of watermarking
If we want to design a watermarking system to protect our

medium we must agree on numerous requirements [2].

Acta Polytechnica Vol. 46 No. 4/2006

Robust Watermarking of Video Streams
T. Polyák

In the past few years there has been an explosion in the use of digital video data. Many people have personal computers at home, and with the
help of the Internet users can easily share video files on their computer. This makes possible the unauthorized use of digital media, and
without adequate protection systems the authors and distributors have no means to prevent it.
Digital watermarking techniques can help these systems to be more effective by embedding secret data right into the video stream. This makes
minor changes in the frames of the video, but these changes are almost imperceptible to the human visual system. The embedded information
can involve copyright data, access control etc. A robust watermark is resistant to various distortions of the video, so it cannot be removed
without affecting the quality of the host medium. In this paper I propose a video watermarking scheme that fulfills the requirements of a
robust watermark.

Keywords: Robust watermarking, video streaming.

Fig. 1: Block scheme of watermarking

Robustness: this refers to the resistance of the watermark to
various distortions and/or attacks. There are many types of
attacks that can impact a watermark: clipping, filtering, noise
adding, etc. The watermark should not be deleted without
significant loss of quality.

Imperceptibility: the watermark cannot be seen. This means
that it can only make changes that are almost invisible to the
human visual system, and it should not affect the quality of
the video very much.

Payload: different applications need watermarks of differ-
ent sizes. For copy protection, one bit is usually enough, but if
we want to store copyright information, we need more bits.

The requirements mentioned above are slightly conflict-
ing: if we maximize robustness, it will worsen imperceptibility,
if we maximize imperceptibility, we can only hide a few bits.
So we have to find the correct relationship between them
(Fig. 2).

3 Overview of the algorithm

The algorithm is designed to embed a watermark in small
resolution (176×144 pixels) video files that can be played
on mobile devices. These devices have limited computational
capacities, so the algorithm should not be too complex.

The algorithm is based on the Dittmann algorithm, which
is designed to watermark MPEG video files [3]. According to
the MPEG DCT coding, Dittmann uses 8×8 pixel blocks to
hide information. Our algorithm better fits more to the needs
of mobile devices.

To improve robustness the algorithm uses larger blocks
than Dittmann’s algorithm. According to the key, different
pattern blocks are added to or subtracted from the selected
blocks of video frames. These pattern blocks are not counted
in real time (to reduce the calculation requirements), they are
read from a file.

The embedding algorithm obtains the following data as
input. These are the free parameters of the algorithm.

The stego key.
The information to embed.
The strength of the watermark (n): this shows the amount

by which the pixel luminance values are modified. Obviously,
if we select a high value we improve robustness but worsen
quality, and vice versa.

Pattern blocks: these are 0–1 blocks, and we hide the
information with the help of these. First they are filled with
random 0–1 values, then we eliminate the high frequencies.
After this, there will be large 0 and 1 islands, so that during
watermark detection the patterns can be more easily recog-
nized, and the watermark will be more resistant to attack.

3.1 Embedding process
Each frame of the video is watermarked. When we embed

the watermark in the frame, first we select the blocks of the
frame which will contain the information bits (each block
holds one bit of information) and the pattern blocks used
for it. Information is hidden in the luminance values of the
pixels. The method is as follows:

Counting the values of the pattern block: where the value
of the chosen pattern is 1, we select n (n is the strength of the
watermark), and where it is 0, the selected value is –n.

The scaled patterns are added to the frame blocks: if we
want to hide 1 in the block, then we add the pattern to it. If
we want to hide 0, we subtract it. The characteristics of the
embedded watermark are shown in Fig. 3. There are eight
watermarked blocks in the frame. High strength (n) is used
for better visibility.

To improve robustness, the same data is hidden in five
frame blocks. The algorithm also places synchronization in-
formation, so it can resist attacks in the time domain, e.g.,
frame dropping.

3.2 Detection process
During detection, the same blocks and pattern blocks are

chosen as in the embedding process. Then the algorithm
reads the bits of the blocks. It summarizes the pixel values
according to the pattern (if the pattern value is 1, the pixel
value is added to the sum, if it is 0, then it is subtracted from
it). Due to the correlation of the bits, the sum should be
around 2n or –2n. If the sum is positive, the hidden bit was 1,
otherwise it was 0.

4 Test results
The tests of the watermark were of two main types: imper-

ceptibility tests and robustness tests.
In the imperceptibility tests I studied the effect of water-

marks of various strengths on the quality of the video. I put
the watermark in the video stream, compared it to the origi-
nal, and calculated the PSNR value caused by the watermark.
Sample frames can be seen in Figs. 4–7, and the test results in
Fig. 8.

In the robustness tests I submitted the watermarked vid-
eos to various attacks and distortions (cropping, filtering,
noise adding, format conversions, etc.) For the strength of
the watermarks I used values 1, 3, 5, 8 and 10. After attacking

50 © Czech Technical University Publishing House http://ctn.cvut.cz/ap/

Acta Polytechnica Vol. 46 No. 4/2006

Fig. 2: Blockscheme of watermarking

Fig. 3: Effect of the watermark on a grey frame

the images I examined how many bits were read correctly by
the detection algorithm. Some test results are shown in Fig. 9.

5 Summary
The test results show that there are appropriate settings of

the algorithm which can ensure robustness of the watermark
and low loss of the original quality of the video. The algo-
rithm is quite simple, it does not use complex calculations, so
it can be used in mobile devices.

References
[1] Furht, B., Kirovski, D.: Multimedia Security Handbook,

CRC PRESS, 2005.
[2] Hartung, F., Kutter., M.: Multimedia watermarking

techniques. Proceeding IEEE, Vol. 87 (1999), No. 7,
p. 1079–1107.

[3] Dittmann, J., Stabenau, M., Steinmetz, R.: Robust MPEG
video watermarking technologies, 1998.

Tamás Polyák
e-mail: polyak.tamas@tmit.bme.hu

Dept. of Telecommunication and Mediainformatics

Budapest University of Technology and Economics,
Műegyetem rkp. 3–9
Budapest, Hungary

Acta Polytechnica Vol. 46 No. 4/2006

Fig. 4: Original video

Fig. 5: Watermarked video (n=3)

Fig. 6: Watermarked video (n=5)

Fig. 7: Watermarked video (n=10)

Fig. 8: PSNR values of video streams watermarked with different
strength

Fig. 9: Correct bit ratio after attacks (adaptive median filtering,
downsampling, format conversion)