DIGITAL WATERMARKING USING MULTIRESOLUTION ANALYSIS
AVIJIT DAS PALLAVI BISWAS SOUMYA CHATTERJEE SUVRA SHANKHA DUTTA
THE WORLD IS STEPPING OVER TO THE DIGITAL MEDIA ADVANTAGE : Transmissions over digital channels provide high noise immunity, and efficient utilization of channel capacity through multiplexing. Digital systems and channels are highly cost effective.
DISADVANTAGE : The ease of accessibility of digital media and the simplicity of the digital systems has rendered the contents over the digital media highly insecure. Digital entities can be easily duplicated, manipulated, or even tampered with. Thus the question of copyright associated with a digital entity faces a severe threat from hackers. 10/19/08 04:02
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DIGITAL WATERMARKING IS THE SOLUTION
The technique of digital watermarking is one of the growing
fields in which this problem of copyright has been addressed elegantly.
The digital watermark is a secret code or image hidden
inside the original image, so as to claim for the copyright of that image.
Thus the process by which the copyright information is
embedded invisibly inside the original entity, which is to be protected from the illegal replication and distribution is known as “Digital Watermarking”.
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Properties of Watermarks Imperceptibility The watermark should not be
noticeable to the viewer, nor should then watermark degrade the quality of the original image. Robustness The watermark must be difficult to
remove. If only partial knowledge is available (e.g. the exact location of the watermark in an image is unknown) then attempts to remove or destroy a watermark, should result in severe degradation in fidelity before the watermark is lost. 10/19/08 04:02
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Common signal processing The watermark should still be
retrievable even if common signal processing operations are applied to the data. These include digital to analog and analog to digital conversion, re-sampling and re-quantization and common signal enhancements to image contrast and color, or audio bass and treble. Common Geometric Distortions Watermarks in image and
video data should also be immune from geometric image operations such as rotation, translation, cropping and scaling. Subterfuge Attacks: Collusion & Forgery if a digital
watermark is to be used in litigation, it must be impossible for colluders to combine their images to generate a different valid watermark with the intention of framing a third party.
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Practical model for a Watermarking System
W
I
IW
WATERMARK INSERTION BLOCK
WATERMARK EXTRACTION BLOCK <W> ATTACK
K
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HACKERS
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ATTACKS ON WATERMARKING SYSTEMS JPEG and MPEG Compression Filtering Rescaling Cropping Jitter Attack
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SPATIAL DOMAIN VS TRANSFORM DOMAIN In the spatial domain the watermark is embedded in
the original image by suitably modifying the gray values of the individual pixels in the letter. Cox and Boland introduced the transform domain
watermarking schemes. Cox used spread spectrum technique to embed the signal bits in the image.
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SPATIAL DOMAIN
ADVANTAGE
Image quality due to watermarking could be controlled by locally baring the region of interest unaffected. Moreover other important higher order factor such as size, shape, color, location and foreground/background of the cover image may be exploited to obtain perceptually tuned and robust watermarking scheme. However, these factors are well specified in the spatial domain and not easily converted to the frequency domain.
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DISADVANTAGE
It is easy to implement on computational point of view but too fragile to withstand large variations of external attacks.
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TRANSFORM DOMAIN Most of the schemes developed for data hiding have embedded
bits in some transform domain (DCT, DFT, Sub-band, Hadamard, Hartley etc.), as it has always been implicitly understood that a decomposition would help. Among different orthogonal decomposition techniques, it was probably the inspiration from the image compression application that caused DCT and sub-band (wavelet) transforms to be more popular than the others. Another reason for the choice of DCT and wavelet based scheme is to “Match” the data-hiding scheme with the processing the image is most likely to undergo. DCT based JPEG, and the wavelet based SPIHT/EZW coding technique is most commonly used. 10/19/08 04:02
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WHY WAVELET TRANSFORM ??
By Fourier Transform we can find the frequency content of a signal. But no frequency information is available in the space (time)- domain signal, and no time information is available in the Fourier transformed signal.
Again the spectrums of stationary and non stationary signals (both of the signals involving the same frequency components) are almost identical, although the corresponding space (time)-domain signals are not even close to each other, but the first one has these frequencies at all times, the second one has these frequencies at different intervals. So how come the spectrums of two entirely different signals look very much alike? Therefore, FT is not a suitable technique for non-stationary signals.
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Because of this reciprocal relationship between scale and frequency , it follows that Wavelet Transform provides better frequency resolution at lower end of frequency spectrum and poor space (time) resolution and vice versa.
This ability to provide variable space (time)-frequency resolution makes the wavelet transform a natural tool in analysis of signal in which rapidly varying high frequency components are superimposed on slowly varying low frequency components.
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Multi Resolution Analysis And Discrete Wavelet Transform Multi Resolution Analysis provides a
framework for understanding of wavelet transform and also for construction of wavelet function. The basic principle is that a signal can be approximated at different resolutions.
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Proposed Watermarking Technique
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Wavelet Transform of Cover Image
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LZW COMPRESSION OF WATERMARK IMAGE b a b a c b a b a b a b c b
21434 857 10/19/08 04:02
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DATA HIDING TECHNIQUE The watermark is embedded in one part of the decomposed image. For hiding one integer of the compressed watermark, the integer is represented as 8 bit binary. And this 8 bit binary pattern is stored in 8 contiguous pixels. Each bit of information is stored in the LSB position of the pixel. A header containing information is also hidden in the part of the image.
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Inverse Wavelet Transform generates the Watermarked Image.
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Watermark Extraction
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LZW DECODER 2 1 4 3 4 8 5 7
babacbabababcb 10/19/08 04:02
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RESULTS
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Special Thanks To Mr. Tirtha Sankar Das and Mr. Somanth Maiti (Head of the Department)
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THANK YOU.
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