VIEWS: 13 PAGES: 4 POSTED ON: 6/1/2011 Public Domain
An Adaptive DCT Domain Visible Watermarking Technique for Protection of Publicly Available Images Saraju P. Mohanty∗ K.R. Ramakrishnan Mohan S Kankanhalli Dept. of Comp. Sc. & Engg. Dept. of Electrical Engg. School of Computing University of South Florida Indian Institute of Science National Univ. of Singapore Tampa, FL 33620, USA Bangalore 560 012, India Kent Ridge, Singapore 119260 smohanty@csee.usf.edu krr@ee.iisc.ernet.in mohan@comp.nus.edu.sg Abstract - At present, with the growth of computer networks and labor intensive than purchasing the image and information technology there is a trend to move from conventional libraries to digital libraries. In the digital from the owner. libraries images and texts are made available through the • The watermark should be applied internet for scholarly research. At the same time care is taken automatically with little human intervention to prevent the unauthorized use of the images commercially. For this purpose the owner needs to use visible watermarking. and labor. In this paper, we describe a visible watermarking scheme that is applied into the host image in the DCT domain. A There are very few visible watermarking mathematical model has been developed for that purpose. techniques available in current literature. The IBM digital library organization has used a I. INTRODUCTION visible watermarking technique to mark the digitized pages of manuscript from the Vatican Digital watermarking is defined as a process of archive [3][9]. Kankanhalli et al. [4] have embedding data (watermark) into a multimedia proposed a visible watermarking technique in object to help to protect the owner's right to that DCT domain. They divide the image into object. The embedded data (watermark) may be different blocks, classify the blocks by perceptual either visible or invisible. In visible watermarking methods proposed in [5] and modify the DCT of images, a secondary image (the watermark) is coefficients of host image as follows. embedded in a primary (host) image such that c'ij(n) = αn cij(n) + βn wij(n) n = 1,2… (1) watermark is intentionally perceptible to a human The αn and βn coefficients are for block n. The observer. cij(n) are the DCT coefficients of the host image block and wij(n) the DCT coefficients of the Some of the desired characteristics of visible watermark image block. watermarks are listed below [1][2]. • A visible watermark should be obvious in In this paper, we propose a visible watermarking both color and monochrome images. technique that modifies the DCT coefficients of • The watermark should be spread in a large or the host image using eqn.(1). But, the αn and βn important area of the image in order to values are found out using a mathematical model prevent its deletion by clipping. developed by exploiting the texture sensitivity of • The watermark should be visible yet must not the human visual system (HVS). This ensures that significantly obscure the image details the perceptual quality of the image is better beneath it. preserved. We call αn the scaling factor and βn as • The watermark must be difficult to remove; the embedding factor. We have also proposed a removing a watermark should be more costly modification to make the watermark more robust. II. FINDING THE SCALING AND • The αn and βn for edge blocks are taken to be EMBEDDING FACTORS αmax and βmin respectively. • For non-edge blocks αn and βn are computed While finding the scaling factors (αn) and as: embedding factors (βn), the following are taken αn = σ'n exp. ( - (µ'n - µ' )2 ) (2) into consideration [4][5][6][7] so that the quality βn = (1/σ n) (1 – exp. ( - (µ n - µ ) )) (3) ' ' ' 2 of the watermarked image is not degraded. where, µ'n, µ' are the normalized values of µn • The edge blocks should be least altered to and µ respectively, and σ'n is normalized avoid significant distortion of the image. So logarithm of σn (the variance of the AC DCT one can add only small amount of watermark coefficients). gray value in the edge block of host image. • αn and βn are then scaled to the ranges (αmin , This means that scaling factor αn should be αmax ) and (βmin , βmax) respectively, where close to αmax, (the maximum value of the αmin and αmax are the minimum and maximum scaling factor) and embedding factor βn values of the scaling factor, and βmin and βmax should be close to βmin, (the minimum value are the minimum and maximum values of the of the embedding factor). embedding factor. These are the parameters • The distortion visibility is low when the determining the extent of watermark background has strong texture. In a highly insertion. textured block, energy tends to be more evenly distributed among the different AC We divide the original image I into 8x8 blocks DCT coefficients. That means AC DCT and find the DCT coefficients of each block. Let coefficients of highly textured blocks have us denote the DCT coefficients of block n by, small variances and we can add more to those cij(n) = 1,2, ... N, where n represents the position blocks. So for convenience, we assume αn to of block in image I (if we traverse the image in a be directly proportional to variance (σn) and raster-scan manner). N is the total number of 8x8 βn to be inversely proportional to variance blocks in the image and given by (row x col)/64, (σn). "row" is the number of rows and "col" is the • Let us denote the mean gray value of each number of columns of the image. image block as µn and that of the image as µ. The blocks with mid-intensity values (µn ≈ µ) The normalized mean gray value of block n is are more sensitive to noise than that of low found out using eqn.(4): intensity blocks (µn < µ) as well as high µ'n = c00(n) / c00max (4) intensity blocks (µn > µ). This means that αn where, c00max is the maximum value of c00(n). should increase with µn as long as (µn < µ) The normalized mean gray value of the image I is and should decrease with µn as long as (µn > calculated using eqn.(5): µ). For convenience, the relationship between µ’ = (1/N) ΣNn=1 c00(n) (5) αn and µn is taken to be truncated Gaussian. The variance of the AC DCT coefficients (σn) of block n is found using eqn.(6): The variation of βn with respect to µn is the σn = (1/64) ΣiΣj (cij - µnAC)2 (6) reverse of that of αn. The mean gray value of each block is given by its DC DCT where, µnAC is the mean of the AC DCT coefficient. coefficients. To confirm to the above requirements we have The normalized variance of the AC DCT coefficients of block n is of the value given by chosen αn and βn as follows. eqn.(7). Let us denote the natural logarithm of σn as σ∗n. σ'n = σ∗n / σ∗max (7) ∗ ∗ where, σ max is the maximum value of σ n. III. INSERTION OF WATERMARK The steps for watermark insertion are discussed now. Fig.2: Original "Lena" • The original image I (to be watermarked) and the watermark image W are divided into blocks of size 8x8. (Both the images may not be of equal size). • The DCT coefficients for each block of the original image are found out. • For each block of the original image I, the normalized mean gray value µ’n is computed using eqn.(4) and are scaled to the range 0.1- 1.0. The normalized image mean gray value Fig.3: Watermarked "Lena" µ is found out using equation (5). (watermark over the whole image) • For the AC DCT coefficients, the normalized variances σ’n are computed using equation (7) and scaled to the range 0.1-1.0. • The edge blocks are identified using the Sobel edge operator. • The αn and βn are found by using equations (2) and (3). • The DCT of watermark image blocks are found out. The nth block DCT coefficient of the host image I is modified using eqn.(1). Fig 4: Watermarked “Lena” The IDCT of modified coefficients give the (small watermark at the corner) watermarked image. IV. MODIFICATIONS TO MAKE THE WATERMARK MORE ROBUST The algorithm proposed here and also that of the classification schemes proposed in [4] are not robust for images having very few objects and Fig.1: Image used as Watermark large uniform areas like in Fig.5. In [4] most of the blocks will be classified to be in one class for Fig.1 shows the image used as watermark. Fig.2 this type of image. If the algorithm discussed in shows the original 'Lena' image. Fig.3 and Fig.4 Section III is applied then most of the blocks will show the watermarked 'Lena’ image with have the same αn and βn values. So in either of different sizes of watermarks. the cases, it is easy for a digital thief to remove the watermark from the watermarked image as it [2] Mintzer F., et al., "Effective and Ineffective would be easy to predict the αn and βn values. We Digital Watermarks", Proc. of IEEE have proposed a modification to our above International Conference on Image watermark insertion technique in [10]. Processing ICIP-97, Vol.3, pp. 9-12, 1997. [3] Mintzer F., et al., "Towards Online Worldwide Access to Vatican Library Materials", IBM Journal of Research and Development, Vol.40, No.2, pp.139-162, Mar. 1996. [4] Kankanhalli M.S., et al., "Adaptive Visible Watermarking of Images", appeared in Proc. of ICMCS'99, Florence, Italy, June 1999. [5] Tao B. and Dickinson B., "Adaptive Fig.5: 'hardware' image Watermarking in DCT Domain", Proc. of IEEE International Conf. on Acoustics, V. CONCLUSIONS Speech and Signal Processing, ICASSP-97, Vol.4, pp.1985-2988, 1997. A visible watermarking technique has been [6] Granrath D.J., "The Role of Human Visual proposed in the DCT domain. A mathematical Models in Image Processing", Proceedings of model has been developed for this purpose the IEEE, Vol.69, No.5, pp.552-561, May exploiting the texture sensitivity of the HVS. We 1981. have also proposed a modification to increase the [7] Kankanhalli M.S., et al., "Content Based robustness of the watermark when used for Watermarking for Images", Proc. of 6th ACM images with very few objects. For more International Multimedia Conference, ACM- robustness, the watermark should not be made MM 98, Bristol, UK, pp.61-70, Sep. 1998. available publicly; the watermark should be used [8] Macq B.M, Quisquater J.J, "Cryptography for in different sizes and should be put in different Digital TV Broadcasting", Proceedings of the portions for different images. The typical values IEEE, Vol.83, No.6, pp.944-957, June 1995. of αmin, αmax, βmin and βmax are 0.95, 0.98, 0.07 [9] Braudaway G. W., Magerlein K. A. and and 0.17 respectively. The visible watermark can Mintzer F., “Protecting Publicly Available be used in digital TV [8], digital library, e- Images with a Visible Image Watermark”, commerce [1][2] etc. Proc. of International Conference on Image Processing, 1997, Vol.1, pp. 524 -527. REFERENCES [10] Mohanty S.P., et al., "A DCT Domain Visible Watermarking Technique for Images", to [1] Yeung M.M., et al., "Digital Watermarking appear in Proc. of the IEEE International for High- Quality Imaging", Proc. of IEEE Conference on Multimedia and Expo, July 30 First Workshop on Multimedia Signal - August 2, 2000, Hilton New York & Processing, Princeton, NJ, pp. 357-362, June Towers, New York City, NY, USA. 1997. ∗ This work was done when the author was at the Indian Institute of Science, Bangalore, India