01-06-2012, 02:07 PM
Combined DWT-DCT Digital Image Watermarking
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INTRODUCTION
The development of effective digital image
copyright protection methods have recently become an
urgent and necessary requirement in the multimedia
industry due to the ever-increasing unauthorized
manipulation and reproduction of original digital
objects. The new technology of digital watermarking
has been advocated by many specialists as the best
method to such multimedia copyright protection
problem[1,2]. Its expected that digital watermarking
will have a wide-span of practical applications such as
digital cameras, medical imaging, image databases, and
video-on-demand systems, among many others[3].
In order for a digital watermarking method to be
effective it should be imperceptible, and robust to
common image manipulations like compression,
filtering, rotation, scaling cropping, collusion attacks
among many other digital signal processing operations.
Current digital image watermarking techniques can be
grouped into two major classes:
THE DCT AND DWT TRANSFORMS
The DCT and DWT transforms have been
extensively used in many digital signal processing
applications. In this section, we introduce the two
transforms briefly, and outline their relevance to the
implementation of digital watermarking.
The DWT transform:
Wavelets are special functions
which, in a form analogous to sines and cosines in
Fourier analysis, are used as basal functions for
representing signals[7]. For 2-D images, applying DWT
corresponds to processing the image by 2-D filters in
each dimension. The filters divide the input image into
four non-overlapping multi-resolution sub-bands LL1,
LH1, HL1 and HH1. The sub-band LL1 represents the
coarse-scale DWT coefficients while the sub-bands
LH1, HL1 and HH1 represent the fine-scale of DWT
coefficients. To obtain the next coarser scale of
wavelet coefficients, the sub-band LL1 is further
processed until some final scale N is reached. When N
is reached we will have 3N+1 sub-bands consisting of
the multi-resolution sub-bands LLN and LHx, HLx and
HHx where x ranges from 1 until N.
PERFORMANCE EVALUATION
We evaluated the performance of the combined
DWT-DCT image watermarking algorithms using a
512´512 'Lena' as the original cover host image, and a
256´256 grey-scale image of the expression 'copyright'
as the watermark image. The two images are shown in
Fig. 4 and 5, respectively.
RESULTS AND DISCUSSION
We described the performance of the combined
DWT-DCT watermarking algorithm. For the sake of
comparison, we also evaluated the watermarking
performance when DWT-Only was used. The results
we obtained for the DWT-Only approach indicated a
better imperceptibility performance was obtained when
the watermark was embedded in the HL2 or HH2 subbands.
The robustness performance, however, was not
acceptable. To improve performance, we combined
DWT with the another equally powerful transform; the
DCT.