A DFT-DWT Domain Invisible Blind Watermarking Techniques for Copyright Protection of Digital Images

The great explosion in this technology has also brought some problems beside its advantages. The great facility in copying a digital content rapidly, perfectly and without limitations on the number of copies has resulted the problem of copyright protection. Digital watermarking is proposed as a solution to prove the ownership of digital data. A watermark, a secret imperceptible signal, is embedded into the original data in such a way that it remains present as long as the perceptible quality of the content is at an acceptable level. The owner of the original data proves his/her ownership by extracting the watermark from the watermarked content in case of multiple ownership claims


Introduction
The great success of Internet and digital multimedia technology have made the fast communication of digital data, easy editing in any part of the digital content, capability to copy a digital content without any loss in quality of the content and many other advantages.
The great explosion in this technology has also brought some problems beside its advantages.The great facility in copying a digital content rapidly, perfectly and without limitations on the number of copies has resulted the problem of copyright protection.Digital watermarking is proposed as a solution to prove the ownership of digital data.A watermark, a secret imperceptible signal, is embedded into the original data in such a way that it remains present as long as the perceptible quality of the content is at an acceptable level.The owner of the original data proves his/her ownership by extracting the watermark from the watermarked content in case of multiple ownership claims In general, any watermarking scheme (algorithm) consists of three parts.

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The watermark.

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The decoder and comparator (verification or extraction or detection algorithm).
Each owner has a unique watermark or an owner can also put different watermarks in different objects the marking algorithm incorporates the watermark into the object.The verification algorithm authenticates the object determining both the owner and the integrity of the object [1].

Embedding process
Let us denote an image by I, a signature by S =s 1 , s 2 , … and the watermarked image by I′.E is an encoder function, it takes an image I and a signature S, and it generates new image which is called watermarked image I′, mathematically, It should be noted that the signature S may be dependent on image I.In such cases, the encoding process described by (1) still holds.The figure 1 illustrates the encoding process [1].

Extraction process
A decoder function D takes an image J (J can be a watermarked or un-watermarked.image, and possibly corrupted) whose ownership is to be determined and recovers a signature S from the image [1].
In this process an additional image I can also be included which is often the original and unwatermarked version of J.This is due to the fact that some encoding schemes may make use of the original images in the watermarking process to provide extra robustness against intentional and unintentional corruption of pixels.Mathematically, In proposed algorithm, original image is not used while extracting watermark from watermarked image and we provide robustness by using some keys.
The extracted signature S will then be compared with the owner signature sequence by a comparator function C δ and a binary output decision generated.It is 1 if there is match and 0 otherwise, which can be represented as follows.
  Where C is the correlator, x= c 3 (S,S').C is the correlation of two signatures and δ is certain threshold.Without loss of generality, watermarking scheme can be treated as a three-tupple (E, D, C δ ).Following figure 2 & figure 3 demonstrate the decoder and the comparator respectively.
A watermark must be detectable or extractable to be useful.Depending on the way the watermark is inserted and depending on the nature of the watermarking algorithm, the method used can involve very distinct approaches.In some watermarking schemes, a watermark can be extracted in its exact form, a procedure we call watermark extraction.In The proposed technique extract watermark to prove ownership.
The quality of extracted watermark can also be measured by: PSNR (Peak Signal-to-Noise Ratio) and AR (Accuracy rate) PSNR is provided only to give us a rough approximation of the quality of the watermark.
Where MSE is mean square error of an image with H × W pixels is defined as: Where ij a is the original pixel value and ij a is the processed pixel value.
Besides, we utilized the accuracy rate AR t o e v a l u a t e t h e r o b u s t n e s s o f a c o p y r i g h t protection scheme for a specific attack.The formula for AR is shown below: Where NP is the number of pixels of the watermark image and CP is the number of correct pixels in the extracted watermark image.

Classification of watermarking techniques
Watermarking techniques can be divided into four categories according to the type of document to be watermarked as follows [1]: Text Watermarking, Image Watermarking, Audio Watermarking and Video Watermarking.
In the case of images from implementation point of view, watermarks can be applied in spatial domain and in frequency domain.In Spatial domain, pixels of one and two randomly selected subsets of an image are modified based on perceptual analysis of the original image.In Frequency domain, values of certain frequencies are altered from their original.
According to human perception, digital watermarks can be divided into three categories as follows [2]: Visible, Invisible-robust and Invisible-Fragile.Visible watermark is where the secondary translucent overlaid into the primary content and appears visible on a careful inspection.Invisible-Robust watermark is embedded is such a way changes made to the pixel value are perceptually unnoticed.Invisible -Fragile watermark is embedded in such a way that any manipulation of the content would alter or destroy the watermark.Sometimes another watermarking called dual watermarking is used.Dual watermark is a combination of a visible and an invisible watermark [1].In this type of watermark an invisible watermark is used as a back up for the visible watermark as clear from the following diagram.Current digital image watermarking techniques can be grouped into two major classes: spatial-domain and frequency-domain watermarking techniques [3].Compared to spatial domain techniques [4], frequency-domain watermarking techniques proved to be more effective with respect to achieving the imperceptibility and robustness requirements of digital watermarking algorithms [5].

Discrete Fourier transform
The Discrete Fourier Transformation (DFT) controls the frequency of the host signal.Energy of watermarking message can be distributed averagly in space domain after the signal is implemented DFT.It enables the schemes further to embed the watermark with the magnitude of its coefficients.
The DFT is useful for watermarking purposes because it helps in selecting the adequate parts of the image for embedding, in order to achieve the highest invisibility and robustness.

The wavelets transform
Wavelet transform decomposes an image into a set of band limited components which can be reassembled to reconstruct the original image without error.The DWT (Discrete Wavelet Transform) divide the input image into four non-overlapping multi-resolution sub-bands LL1, LH1, HL1 and HH1.The process can then be repeated to computes multiple "scale" wavelet decomposition, as in the 2 scale wavelet transform shown in Fig. 6.
One of the many advantages over the wavelet transform is that it is believed to more accurately model aspects of the HVS as compared to the FFT or DCT.This allows us to use higher energy watermarks in regions that the HVS is known to be less sensitive, such as the middle frequency bands (LH, HL) and high resolution band (HH).But watermark embedded in high resolution band can be easily be distorted by geometric transformation, compression and various signal processing operations.
Embedding watermarks in middle frequency regions allow us to increase the robustness of our watermark, at little to no additional impact on image quality [6].

Watermarking applications
Although the main motivation behind the digital watermarking is the copyright protection, its applications are not that restricted.There is a wide application area of digital watermarking, including broadcast monitoring, fingerprinting, authentication and covet communication [7,8,9,10].
By embedding watermarks into commercial advertisements, the advertisements can be monitored whether the advertisements are broadcasted at the correct instants by means of an automated system [7,8].The system receives the broadcast and searches these watermarks identifying where and when the advertisement is broadcasted.The same process can also be used for video and sound clips.Musicians and actors may request to ensure that they receive accurate royalties for broadcasts of their performances.
Fingerprinting is a novel approach to trace the source of illegal copies [7,8].The owner of the digital data may embed different watermarks in the copies of digital content customized for each recipient.In this manner, the owner can identify the customer by extracting the watermark in the case the data is supplied to third parties.The digital watermarking can also be used for authentication [7,8].The authentication is the detection of whether the content of the digital content has changed.As a solution, a fragile watermark embedded to the digital content indicates whether the data has been altered.If any tampering has occurred in the content, the same change will also occur on the watermark.It can also provide information about the part of the content that has been altered.
Covert communication is another possible application of digital watermarking [7,8].The watermark, secret message, can be embedded imperceptibly to the digital image or video to communicate information from the sender to the intended receiver while maintaining low probability of intercept by other unintended receivers.

A DFT-DWT Domain Invisible Blind Watermarking Techniques for Copyright Protection of Digital Images 521
There are also non-secure applications of digital watermarking.It can be used for indexing of videos, movies and news items where markers and comments can be inserted by search engines [8].Another non-secure application of watermarking is detection and concealment of image/video transmission errors [11].For block based coded images, a summarizing data of every block is extracted and hidden to another block by data hiding.At the decoder side, this data is used to detect and conceal the block errors.

Watermarking requirements
The efficiency of a digital watermarking process is evaluated according to the properties of perceptual transparency, robustness, computational cost, bit rate of data embedding process, false positive rate, recovery of data with or without access to the original signal, the speed of embedding and retrieval process, the ability of the embedding and retrieval module to integrate into standard encoding and decoding process etc. 7,8,9,12,13].
Depending on the application, the properties, which are used mainly in the evaluation process, varies.
The main requirements for copyright protection are imperceptibility and robustness to intended or non-intended any signal operations and capacity.
The owner of the original data wants to prove his/her ownership in case the original data is copied, edited and used without permission of the owner.In the watermarking research world, this problem has been analyzed in a more detailed manner [13,14,15,16,17,18].
The imperceptibility refers to the perceptual similarity between the original and watermarked data.The owner of the original data mostly does not tolerate any kind of degradations in his/her original data.Therefore, the original and watermarked data should be perceptually the same.Robustness to a signal processing operation refers to the ability to detect the watermark, after the watermarked data has passed through that signal processing operation.
The robustness of a watermarking scheme can vary from one operation to another.Although it is possible for a watermarking scheme to be robust to any signal compression operations, it may not be robust to geometric distortions such as cropping, rotation, translation etc.The signal processing operations, for which the watermarking scheme should be robust, changes from application to application as well.While, for the broadcast monitoring application, only the robustness to the transmission of the data in a channel is sufficient, this is not the case for copyright protection application of digital watermarking.For such a case, it is totally unknown through which signal processing operations the watermarked data will pass.Hence, the watermarking scheme should be robust to any possible signal processing operations, as long as the quality of the watermarked data preserved.
The capacity requirement of the watermarking scheme refers to be able to verify and distinguish between different watermarks with a low probability of error as the number of differently watermarked versions of an image increases [17].While the robustness of the watermarking method increases, the capacity also increases where the imperceptibility decreases.There is a trade off between these requirements and this trade off should be taken into account while the watermarking method is being proposed.

Proposed techniques
We have proposed a blind invisible watermarking technique for copyright protection of the colored images.
In blind techniques, during the extraction process original image is not required.Watermarking systems which involve marking imperceptible alteration on the cover data to convey the hidden information, is called invisible watermarking.Here 512*512 grayscale image of 'peppers' is taken as host image and 32*32 binary image is taken as watermark image.
Then implement second level wavelet transform on host image using wavelet function 'haar' and extracted middle level components (HL 2 , LH 2 ) for embedding watermark.Middle level components are selected for embedding watermark as much of the signal energy lies at low-frequencies sub-band which contains the most important visual parts of the image and high frequency components of the image are usually removed through compression and noise attacks.
We have divided the HL 2 and LH 2 bands in to 4x4 blocks and applied DFT in these blocks and used to two highly uncorrelated pseudo random sequences (treated as key: key1) to embed watermarking message according to template matrix.We reshaped watermark image of 32x32 into a row vector of size 1024, called watermark message.
A template matrix is such a matrix whose size is 4x4 and elements are 0 and 1.
Watermarking message is embedded into image blocks only in the position where the template matrix's element is 1.Through amounts of experiments, we found when the template matrix is set to [1,1,1,1;1,0,0,1;0,0,0,1;0,0,0,1] T ,imperceptibility and robustness of the algorithm can get better balance.Here template matrix is used as a key2.
We have embedded watermark according to the (9) given below.
      w I x, y I x, y k W x, y   In ( 9), k denotes a gain factor, and I W the resulting watermarked image, I the cover image and W the watermark to be embedded.Increasing k increases the robustness of the watermark at the expense of the quality of the watermarked image.
The algorithm for the proposed method is given below: The watermark embedding steps are as follows:  Exhaustive testing against signal processing operation, Geometric distortion , collusion still has to be performed.

Conclusion
The need for digital watermarking on electronic distribution of copyright material is becoming more prevalent.In this paper an overview of the digital watermarking techniques are given and a blind invisible watermarking technique for grayscale images based on DWT and DFT is presented.The algorithm use 512*512 gray images as a host image and 32*32 binary image as watermarked image.
Firstly, two level wavelet decomposition is implemented on the host image.Then, the middle frequency components are extracted and divided in to several blocks of size 4*4 and DFT is implemented on them.Finally, two pseudo random sequences are created and embedded to blocks which have implemented DFT according to whether the corresponding position is 0 or 1 in the watermark matrix which has been implemented.
The original image is not required while extracting the watermark Instead, correlations among each block and two sequences are respectively calculated.Watermark is recovered on foundation of the relative magnitude of correlation between the corresponding block and one sequence or the other.The idea of applying two transform is based on the fact that

Fig. 4 .
Fig. 4. Schematic representation of dual watermarkingFrom application point of view, digital watermarking could also be[2]: source based and destination based.In source based a unique watermark identifying the owner is introduced to all the copies of particular content being distributed.Destination based is where each distributed copy gets a unique watermark identifying the particular buyer.Different types of watermarks are shown in the figure.5.

AFig. 5 .
Fig. 5. Types of watermarking techniques Commonly used frequency-domain transforms include the Discrete Wavelet Transform (DWT), the Discrete Cosine Transform (DCT) and Discrete Fourier Transform (DFT).The host signal is transformed into a different domain and the watermark is embedded in selective coefficients.Here we have described DFT and DWT domain techniques.