Image Watermarking



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Figure 3). The normal distribution is used because it has been proven to be quite robust to collusive attacks. In order to weight the watermark according to the magnitude of the wavelet coefficients, we used one of the two following relations between the original coefficients y and ÿ , the ones containing the watermark:

ÿ[m,n] = y[m,n] + alpha (y[m,n])2 N[m,n] ( 1 )

&

ÿ[m,n] = y[m,n] + alpha . abs(y[m,n]) N[m,n] ( 2 )



It must be pointed out that the relations (1) and (2), even though they are mathematically different, have the exact same goal which is to put more weight to the watermark added to high value wavelet coefficients. The parameter alpha is to control

the level of the watermark; it is in fact a good way to choose between good transparency or good robustness or a tradeoff between the two. Finally, the two-dimension inverse wavelet transform of is computed to form ÿ the watermarked image.





Figure 4 gives a good idea of the main components of the encoder
4.2.2 Decoder

At the other end of the communication channel, a decoder is used to extract the watermarked information from the received image. Upon reception of the supposedly watermarked image, the algorithm first isolates the signature included in this image by comparing the DWT coefficients of the image with those of the original (non-watermarked) one. The following operation consists of taking the identified key to put in contrast with the found signature by computing the cross-correlation at the first resolution level (i.e. highest frequency coefficients). The watermark is called detected if there is a peak in the cross-correlation corresponding to a positive identification. If there is no central peak, the decoder adds the second resolution level (i.e. the bottom left square in the pyramid structure of Figure 3) to the computation aiming at finding for a peak. Once again, if there is a peak, the watermark is called detected and if not, we go to the third resolution… and so on until we reach the ninth resolution limit.


4.3 Advantage of this technique

The main advantage of this technique is that while allowing good detection, even in the presence of corruption, it keeps the level of false positive detection to a minimum since the found signature has to go through detection step of positive identification to be called detected. The detector step aims at ensuring the maximum exactitude in the detection of the owner identification key and, as said previously, minimizing the number of false positive detection. The results presented later on should convince the reader of the performance of our decoder.




  1. Applications of Watermarking

5.1 Fingerprinting

In order to trace the source of illegal copies the owner can embed different watermarking keys in the copies that are supplied to different customer. For the owner, embedding a unique serial number-like watermark is a good way to detect customers who break their license agreement by copying the protected data and supplying it to a third party.



5.2 Indexing

Watermarking offers a wide range of new capabilities to multimedia applications. It allows the indexing of video mail by permitting the insertion of comments in video content as well as the indexing of movies or news items by making available the utilization of markers that can be exploited in search engines. As the number of images and video contents online increases a lot faster than the capabilities of today’s search engine, it is important to plan ahead for new ways to allow quick access to multimedia data and watermarking is certainly a promising way to do so.


5.3 Copyright Protection & Owner identification:

To protect its intellectual property, the data owner can embed a watermark representing copyright information of his data. This application can be a really helpful tool in settling copyright disputes in court. It is probably the most widely spread use of digital images watermarking and it is also the application we have worked on in the present project.


5.4 Broadcast monitoring:

In order to help the automated identification of broadcasted programs, original watermarks can be inserted in any type of data to be widely broadcasted on a network. It could assure that advertisers received the airtime they have paid for or make certain that musicians’ property is not rebroadcast by pirate stations (or at least, if so, that it can be detected).


5.5 Copy protection:

The watermarked information can directly control digital recording device. The embedded key can represent a copy-permission bit stream that is detected by the recording device which then decide if the copying procedure should go on (allowed) or not (prohibited).



5.6 Data Authentication:

Fragile watermarks are used to detect any corruption of an image or any other type of data. If the watermark is detected, the data is genuine, if not, the data has been corrupted and cannot be considered.


5.7 Data Hiding (Covert Communications):

The transmission of private data is probably one of the earliest applications of watermarking. As one would probably have already understood, it consists of implanting a strategic message into an innocuous one in a way that would prevent any unauthorized person to detect it.



5.8 Medical Safety:

Embedding the date and patient’s name in medical images could increase the confidentiality of medical information as well as the security.




  1. Requirements of Watermarking

To achieve maximum protection of intellectual property with watermarked media, several requirements must be satisfied:

6.1 Imperceptible

The watermark should be imperceptible so as not to affect the viewing experience of the image or the quality of the audio signal.



6.2 Undeletable

The watermark must be difficult or even impossible to remove by a hacker, at least without obviously degrading the host signal.



6.3 Statistically Undetectable

A pirate should not be able to detect the watermark by comparing several watermarked signals belonging to the same author.



6.4 Robustness

6.4.1 The watermark should be survive the lossy compression techniques like JPEG, which is commonly used for transmission and storage.

6.4.2 The watermark should still be retrievable even if common signal processing operation are applied, such as signal enhancement, geometric image operations, noise, filtering, and etc.

6.5 Unambiguous

Retrieval of the watermark should be unambiguously identify the owner, and the accuracy of identification should degrade gracefully in the face of attacks



  1. Conclusions

As we have witnessed in the past few months, the problem of protecting multimedia information becomes more and more important and a lot of copyright owners are concerned about protecting any illegal duplication of their data or work. Some serious work needs to be done in order to maintain the availability of multimedia information but, in the meantime, the industry must come up with ways to protect intellectual property of creators, distributors or simple owners of such data.
This is an interesting challenge and this is probably why so much attention has been drawn toward the development of digital images protection schemes.
Of the many approaches possible to protect visual data, digital watermarking is probably the one that has received most interest. The goal of digital watermarking is to produce an image that looks exactly the same to a human eye but still allows its positive identification in comparison with the owner's key if necessary.

8. References

[1] I J Cox , M L Miller and J A Bloom , Watermarking Applications and their Properties .

[2] A H Paquet , R K Ward , Wavelet Based Digital Watermarking

[3] G Voyatzis and I Pitas , The Use Of Watermarks in the Protection of Digital Multimedia Products .

[4] H Y Lo , S Topiwala and J Wang , Wavelet Based Steganography and Watermarking .

[5] http://www.webopedia.com/TERM/D/digital_watermark.html





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