A robust image watermarking scheme invariant to rotation, scaling and translation attacks
Abstract
Digital watermarking has been proposed as an effective means of copyright protection, copy
protection and finger
-
prin
ting of digital multimedia. These are all means of protecting the digital
multimedia from illegal copying and/or distribution. The need for protection of digital
multimedia is much more acute than their analogue counterparts. This is because they are
trans
mitted over the internet exposing them to the risk of being copied illegally. Various methods
of digital image watermarking have been proposed. Attackers always attempt to remove the
embedded watermarks or render them useless by using either geometric atta
cks or signal
processing attacks. Most watermarking algorithms are fairly robust to signal processing attacks
but are fragile to geometric attacks.
This study proposes a
watermarking scheme that is robust to geometric distortion attacks and
signal processi
ng attacks without increasing the computational complexity of the algorithm.
The
proposed method is based on
embedding the watermark in the
Discrete
Cosine Transform (DCT)
domain and then employing
a spread
-
spectrum format
to enhance security
and Vector
Qu
anti
zation (VQ) techniques
to compress the image. Since embedding the watermark in the
DCT domain gives it sufficient robustness to signal processing attacks, the proposed techniq
ue
focuses more on
rotation, scaling and translation
(
RST
)
attacks
.
This
meth
od employs
the
Harris
corner detector
-
based feature
-
points to
obtain
a Delaunay tessellation
that
is used in reversing the
geometric attacks before attempting to extract the watermark.
In situations where the RST attacks lead to formation of substantial
ly
dark regions in the image,
some feature
-
points are lost or additional feature
-
points are obt
ained leading to formation of a
different Delaunay tessellation. This results in
a
watermark
of
poor quality or even entirely lost
in some instances.
A
method of i
mproving the estimation of the nature of attack and thereby
leading to an improvement in the quality of the extracted watermark is proposed
in this thesis
.
The
method is based on the
estimation of the RST attack
of an image by taking a mean distortion
on s
elected triangular regions.
The accuracy of the proposed method has been tested by comput
er simulation experiments using
MATLAB
. Various forms of attacks such as
image filtering, image noising, histogram
equalization, image cropping, JPEG compression and R
ST attacks
on standard test images have
been simulated and watermark recovery assessed
. The
test images that have been include the
256
×
256
8
-
bit grey level images ‘Pepper’, ‘Cameraman’ and ‘H
ouse’
and the
512
×
512
8
-
bit
grey level images
‘
L
ifting body’, ‘Gi
rl’ and ‘
Baboon' among
others.
The proposed algorithm has also been tested on colour images such as ‘Baboon’, ‘Couple’ and
‘House’ and on video such as ‘Bus’, ‘Foreman’ and ‘Container’ among others.
The P
eak
S
ignal
-
to
-
N
oise
R
atio (PSNR)
and the Normalised
Cross
-
correlation (NC) measures
have been used to assess the quality of the watermarked images and recovered watermarks.
The computer simulation results show that the proposed algorithm is effective in
extraction of
watermarks from images subjected to geom
etric attacks.
Publisher
University of Nairobi