This document proposes an improved spread spectrum watermarking technique to withstand geometric deformations. The technique uses Direct Sequence Code Division Multiple Access (DS-CDMA) and embeds the watermark by modulating a pseudo-random noise sequence. It normalizes the cover image before embedding to achieve robustness against geometric attacks like rotation and scaling. The watermark is extracted using correlation between the watermarked image and pseudo-random noise sequence. Experimental results show the technique achieves a low bit error rate under various geometric attacks when the watermark strength is increased.

1. ACEEE International Journal on Signal and Image Processing Vol 1, No. 1, Jan 2010
An improved Spread Spectrum Watermarking
technique to withstand Geometric Deformations
A. Sangeetha 1 ,K.Anusudha 2 ,B.Gomathy 3 and K.Surya Tej 4
1
asangeetha@vit.ac.in1
2
Kanusudha@vit.ac.in2
3
gomathy_tc16@yahoo.co.in3
4
kunchesuryatej@gmail.com4
School of Electrical Sciences
VIT University, Vellore-14
Abstract—Here, we propose a new method for the modulated spread spectrum with frequency spectrum,
watermarking to withstand the geometric attacks, which centered at the carrier frequency. The information is
may occur during the transmission of the watermarked demodulated at the receiving end by multiplying the
image. The underlying system is based on Direct Sequence signal by a locally generated version of the pseudo-
Code Division Multiple Access (DS-CDMA). The algorithm
for the normalization has been formulated for use in black
random code sequence. This process, known as "de-
and white images. The watermark is spread across the spreading", mathematically constitutes a correlation of
carrier image by using the pseudo-random noise sequences the transmitted PN sequence with the PN sequence that
of optimal period and retrieval is made by the use of the receiver believes the transmitter is using.
correlation. Private Key technique is used so the
transmission is very secure. Matlab was used to implement
IV. WATERMARKING METHODOLOGY
the algorithm discussed here.
I. INTRODUCTION The original image is taken and converted into gray
Geometric deformations include rotation, scaling, scale if required. Normalization procedure is applied to
translation, shearing, random bending, and change of the original image. A PN sequence is generated using a
aspect ratio (e.g., [1]–[3]). It is well known that a small key element, which is confidential to the organization
amount of rotation and/or scaling can dramatically alone. Create a two-dimensional (2-D) watermark with
disable the receiver from detecting the watermark [4].A the same size as the normalized image. Binary pseudo-
watermark is robust if it cannot be impaired without also random sequences pi, i=1,2,3…. M is generated, as
rendering the attacked data useless. Watermark signature patterns using the private key as seed, where
impairment can be measured by criteria such as miss M is the number of bits in the watermark message.
probability, probability of bit error, or channel capacity. Then the last two digits of the sequence will be XORed
Hence, robustness can be evaluated by simultaneously and the value will be shifted once this process will
considering watermark impairment and the distortion of continue till code of length equal to the length of the
the attacked data. The key idea of this watermarking cover image is generated. A 1-D DS-CDMA
scheme is to use a normalized image for both watermark watermark signature by modulating the watermark
embedding and detection. message with the patterns generated in previous steps
is created.
Message is embedded to the normalized image.
II. WATERMARKING USING CDMA TECHNIQUES Desired watermarking strength is used before
The CDMA technique is a spread spectrum technique addition.A mask image is created, which is a binary
that spreads the transmitted signal over a wide image of the same size as the normalized image. This
frequency band, which is much wider than the actual image has 1s within the support of the normalized
minimum bandwidth required. This technique ensures image and 0s elsewhere. Using the mask image the
the survival of watermark under various attacks due to boundary is masked of if it is excess than the cover
redundancy. image.Inverse normalization is done to this watermark
embedded image. This is the watermarked image and
III. DIRECT SEQUENCE SPREAD SPECTRUM this is transmitted.
In this form of modulation, a pseudo-random noise In the receiver side the image is normalized. Using the
generator creates a high-speed pseudo-noise code same key PN sequence is again generated. Correlation
sequence (sequence of 1 and −1 values). Direct- is performed between the watermarked image and the
sequence spread-spectrum transmissions multiply the PN sequence. Mean of the correlation values are taken
data being transmitted by this "noise" signal; thus, it and a threshold is fixed. Message is decoded using this
directly sets the transmitted radio frequency (RF) threshold.
bandwidth. The result of modulating an RF carrier with
such a code sequence is to produce a direct-sequence-
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2. ACEEE International Journal on Signal and Image Processing Vol 1, No. 1, Jan 2010
V. IMPLEMENTATION So that the resulting image, denoted by, f 3(x, y) = Ay
[f 2(x, y)]. γ Can be calculated using the formula,
The parameters by which the image is normalized are
estimated from the geometric moments of the image
[4]. By putting μ11(3) =0 we get
A. Image Moments and Affine Transforms
Let f (x, y) denote a digital image of size M x N. Its
geometric moments mpq and μpq central moments, p, q
= 0, 1, 2, 3… are defined, respectively as Scale f 4(x, y) in both x and y directions with
As = α 0 so that the resulting image denoted by,
0 β
And
f 4(x,y) = As[f 3(x,y)] achieves
1) A prescribed standard size.
2) μ50(4)>0 and μ05(4)>0.
Where Where, α= Standard image size/number of columns in
y-sheared image.
β=Standard image size/number of rows in y-
An image g (x,y) is said to be an affine transform of sheared image.
f(x,y) if there is a matrix A= a11 a21 The final image f4 (x, y) is the normalized image.
a12 a22
and the vector d = d1 such that f(x,y)=g(x,y),
d2
where
B.Normalization procedure
The four steps of normalization are:
Center the image f (x,y); this is achieved by setting
the matrix A= 0 1 and the
1 0
Vector with d= d1 with,
d2
Let f 1(x, y) denotes the resulting centered image.
Apply a shearing transform to f 1(x, y) in the x
direction with matrix Ax = 1 β
0 1
So that the resulting image Figure 1. Block diagram
denoted by, f 2(x,y) = Ax[f1(x,y)].β can be calculated
using the formula, C.Embedding
The addition of the PN sequences to the cover image
In particular, we may have the following two is done according to the equation:
scenarios: Iw (x, y) = I (x, y) + k × W (x, y)
1) One of the three roots is real and the other two This is shown in figure given bellow
are complex, we select the real root Where, Iw (x, y) denotes the watermarked image.
2) All three roots are real, then we pick the I (x, y) denote the actual cover image.
median of the three real roots. W (x, y) denotes a pseudorandom noise pattern that is
added to the image.
Apply a shearing transform to f 2(x, y) in the y K denotes the gain factor.
direction with matrix Ay = 1 0
γ 1
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3. ACEEE International Journal on Signal and Image Processing Vol 1, No. 1, Jan 2010
Figure 2(a) Embedding process step-1
Figure 5(a) watermark message Figure 5(b) watermarked image
This image is masked to remove borders in watermark
message if greater than normalized image. To the
normalized and masked image inverse normalization is
Figure 2(b) Embedding process step-2
done. Inverse normalization involves the steps, which
is simply the inverse of the steps involved in
D.Extraction normalization.
The multiplier output C of figure.3 is given by
C = Iw (x, y) × b (x, y)
= (a(x,y) × b(x,y) + I(x,y)) × b(x,y)
= a(x,y) × b^2(x,y) + I(x,y) × b(x,y)
Figure 6(a) masked image Figure 6(b) image to be transmitted
Figure.3 extraction process
Receiver side results for a watermarking strength K= 2
The watermark image a (x, y) is multiplied twice with
the noise signal b (x, y) which becomes 1 whereas the
unwanted or the cover image I (x, y) is multiplied only
once with the noise signal that can be filtered out
during the process of correlation by setting the
Recovered Watermark
threshold as mean of correlation.
VI. RESULT ANALYSIS
Figure 7(a) received image Figure 7((b)recoverd watermark
The first step is normalization.
This difference image below shows that the technique
ensures high degree of fidelity. As the gain is increased
from 2 to 4, the recovery of the watermark improves,
but at the cost of distorting the watermarked image.
Figure 4(a) original image Figure 4(b) normalized image
Then the watermark is embedded.
Figure 8. Difference image
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4. ACEEE International Journal on Signal and Image Processing Vol 1, No. 1, Jan 2010
VII. ATTACKS A .BER after Geometric Distortion
TABLE I. Comparison between Watermark Recovery with and ∗ Flipping
without Normalization TABLE II. BER for flipping
Flipping BER
Type of attack With normalization Without
Normalization
Horizontal / Vertical 0.0443
Line & column
Removal
∗ Scaling
TABLE III. BER for scaling
Scaling
Scaling BER
0.75 0.0461
0.5 0.0461
Aspect ratio 1.1 0.0461
Change 1.5 0.0425
∗ Aspect ratio change
Shearing
TABLE IV. BER for change of aspect ratio.
Aspect Ratio BER
Affine (1, 0.8) 0.0490
Transformation (1, 0.9) 0.0437
(1, 1.1) 0.0437
Horizontal (1, 1.2) 0.0514
Flipping
∗ Line and column removal
TABLE V. BER for line & column removal
Vertical Flipping
Number of Rows & Columns BER
Median filtering (1, 1) 0.0425
(17, 5) 0.0443
The above shows the watermarking recovery with and
without normalization. From the recovered images it is ∗ Shearing
seen that the normalization procedure resulted in a TABLE VI. BER for shearing
better geometric deformation resistance to the images.
Shearing BER
VIII. BIT ERROR RATIO
Waterm strength Vs BER
ark
(0, 1%) 0.0319
0.2 (5%, 5%) 0.0461
0.15
∗ General geometric affine transformation
BER
0.1 TABLE VII. BER for general geometric affine
transformation
0.05
Matrix BER
0
1 2 3 4 5 6 7 8 9 1.1 0.2 0
Waterm strength
ark -0.1 0.9 0 0.1329
0 0 1
Figure 9. Plot between watermark strength Vs BER
0.9 -0.2 0 0.1010
From the plot we can infer that the Bit Error rate 0.1 1.2 0
0 0 1
decreases with the increase in watermark strength.
1.01 0.2 0 0.0691
-0.2 0.8 0
0 0 1
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5. ACEEE International Journal on Signal and Image Processing Vol 1, No. 1, Jan 2010
IX. CONCLUSION
The proposed algorithm achieves its robustness by both
embedding and detecting the watermark message in the
normalized images. It is demonstrated that the
proposed algorithm can achieve very low decoding
BER when used with multi bit watermarks under
various affine attacks. From the analysis, the gain
factor k=2 is arrived which gives a good balance
between the visual quality and watermark robustness.
The above process provides high security to the
copyright information and preventing access from
unauthorized users.
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© 2010 ACEEE
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