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(IJCSIS) International Journal of Computer Science and Information Security, Vol. 8, No. 2, 2010 IMAGE SUPER RESOLUTION USING MARGINAL DITRIBUTION PRIOR S.Ravishankar Dr.K.V.V.Murthy Department of Electronics and Communication Department of Electronics and Communication Amrita Vishwa Vidyapeetham University Amrita Vishwa Vidyapeetham University Bangalore, India Bangalore, India s_ravishankar@blr.amrita.edu kvv_murthy@blr.amrita.edu Abstract— In this paper, we propose a new technique for image wavelet transform. They estimate the wavelet coefficients at super-resolution. Given a single low resolution (LR) observation higher scale from a single low resolution observation and and a database consisting of low resolution images and their high achieve interpolation by taking in-verse wavelet transform. resolution versions, we obtain super-resolution for the LR The authors in [5] propose technique for super-resolving a observation using regularization framework. First we obtain a close approximation of the super-resolved image using learning single frame image using a database of high resolution images. based technique. We learn high frequency details of the They learn the high frequency details from a database of high observation using Discrete Cosine Transform (DCT). The LR resolution images and obtain initial estimate of the image to be observation is represented using a linear model. We model the super-resolved. They formulate regularization using wavelet texture of the HR image using marginal distribution and use the prior and MRF model prior and employ simulated annealing same as priori information to preserve the texture. We extract for optimization. Recently, learning based techniques are the features of the texture in the image by computing histograms employed for super-resolution. Missing information of the of the filtered images obtained by applying filters in a filter bank high resolution image is learned from a database consisting of and match them to that of the close approximation. We arrive at high resolution images. Freeman et al. [6] propose an example the cost function consisting of a data fitting term and a prior term and optimize it using Particle Swarm Optimization (PSO). based super-resolution technique. They estimate missing high- We show the efficacy of the proposed method by comparing the frequency details by interpolating the input low-resolution results with interpolation methods and existing super-resolution image into the desired scale. The super-resolution is techniques. The advantage of the proposed method is that it performed by the nearest neighbor based estimation of high- quickly converges to final solution and does not require number frequency patches based on the corresponding patches of input low resolution observations. low-frequency image. Brandi et al. [7] propose an example- based approach for video super-resolution. They restore the Keywords-component; formatting; style; styling; insert (key high-frequency words) I.INTRODUCTION Information of an interpolated block by searching in a database for a similar block, and by adding the high frequency In many applications high resolution images lead to better of the chosen block to the interpolated one. They use the high classification, analysis and interpretation. The resolution of an frequency of key HR frames instead of the database to image depends on the density of sensing elements in the increase the quality of non-key restored frames. In [8], the camera. High end camera with large memory storage authors address the problem of super-resolution from a single capability can be used to capture the high resolution images. image using multi-scale tensor voting framework. They In some applications such as wildlife sensor network, video consider simultaneously all the three color channels to produce surveillance, it may not be feasible to employ costly camera. a multi-scale edge representation to guide the process of high- In such applications algorithmic approaches can be helpful to resolution color image reconstruction, which is subjected to obtain high resolution images from low resolution images the back projection constraint. The authors in [9] recover the obtained using low cost cameras. The super-resolution idea super-resolution image through neighbor embedding was first proposed by Tsai and Huang [1]. They use frequency algorithm. They employ histogram matching for selecting domain approach and employ motion as a cue. In [2], the more reasonable training images having related contents. In authors use a Maximum a posteriori (MAP) framework for [10] authors propose a neighbor embedding based super- jointly estimating the registration parameters and the high- resolution through edge detection and Feature Selection resolution image for severely aliased observations. The (NeedFS). They propose a combination of appropriate features authors in [3] describe an MAPMRF based super-resolution for preserving edges as well as smoothing the color regions. technique using blur cue and recover both the high-resolution The training patches are learned with different neighborhood scene intensity and the depth fields simultaneously. The sizes depending on edge detection. The authors in [11] authors in [4] present technique of image interpolation using propose modeling methodology for texture images. They 347 http://sites.google.com/site/ijcsis/ ISSN 1947-5500 (IJCSIS) International Journal of Computer Science and Information Security, Vol. 8, No. 2, 2010 capture the features of texture using a set of filters which is the number of the training sets in the data base. Now the represents the marginal distribution of image and match the best matching HR block for the considered low resolution same in feature fusion to infer the solution. In this paper, we image block (up-sampled) is obtained as 8 m = ������������������������������������������������ � � �CT (i , j) − CLR (i, j)� . (m) (m) 2 propose an approach to obtain super-resolution from a single image. First, we learn the high frequency content of the super- ��������+�������� >��������ℎ������������������������ ℎ������������������������ resolved image from the high-resolution training images in the data base and use the learnt image as a close approximation to � Here, m(i, j) is the index for the training image which gives (1) the final solution. We solve this ill-posed problem using prior information in the form of marginal distribution. We apply different filters on the image and calculate the histograms. We the minimum for the block. Those non aliased best matching assume that these histograms remain deviate from that of the HR image DCT coefficients are now copied in to the close approximation. We show the result of our method on corresponding locations in the block of the up sampled test real images and compare it with the existing approaches. image. In effect, we learn non aliased DCT coefficients for . the test image block from the set of LR-HR images. The coefficients that correspond to low frequencies are not altered. Thus at location (i , j)in a block, we have, �������� (��������, ��������) ���������������� (�������� , ��������) > ��������ℎ������������������������ℎ������������������������ II. DCT BASED APPROACH FOR CLOSE (�������� ) � ΧΤ(ι, ϕ) = � ���������������� � APPROXIMATION. ����������������(�������� ,�������� ) �������������������������������� (2) In this section, DCT based approach to learn high frequency details for the super-resolved for a decimation factor of 2 (q = 2) is described. Each set in the database consists of a pair of This is repeated for every block in the test image. We low resolution and high resolution image. The test image and conducted experiment with different Threshold values. We LR training images are of size M × M pixels. Corresponding begin with Threshold =2 where all the coefficients except the HR training images have size of 2M × 2M pixels. We first up DC coefficient are learned. We subsequently increased the sample the test image and all low resolution training images threshold value and conducted the experiment. The best by factor of 2 and create images of size 2M X 2M pixels each. results were obtained when the Threshold was set to 4 that A standard interpolation technique can be used for the same. correspond to learning a total of 10 coefficients from the best We divide each of the images, i.e. the up sampled test image, matching HR image in the database. After learning the DCT up sampled low resolution images and their high resolution coefficients for every block in the test image, we take inverse versions, in blocks of size4 × 4. The motivation for dividing DCT transform to get high spatial resolution image and into 4X4 block is due to the theory of JPEG compression consider it as the close approximation to the HR image. where an image is divided into 8X8 blocks in order to extract the redundancy in each block. However, in this case we are interested in learning the non aliased frequency components III. IMAGE FORMATION MODEL from the HR training images using the aliased test image and the aliased LR training images. This is done by taking the In this work, we obtain super-resolution for an image from a DCT on each of the block for all the images in the database as single observation. The observed image Y well as the test image. Fig.1.a) shows the DCT blocks of the is of size M x M pixels. Let y represent the lexicographically up sampled test image whereas Fig.1. (b) Shows the DCT ordered vector of size M 2 × 1, which contains the pixels from blocks of up sampled LR training images and HR training image Y and z be the super-resolved image. The observed images. We learn DCT coefficients for each block in the test images can be modeled as image from the corresponding blocks in the HR images in the database. It is reasonable to assume that when we interpolate y = Dz + n, (3) the test image and the low resolution training images to obtain 2M × 2M pixels, the distortion is minimum in the lower where D is the decimation matrix which takes care of aliasing. frequencies. Hence we can learn those DCT coefficients that For an integer decimation factor of q, the decimation matrix D correspond to high frequencies (already aliased) and now consists of q2 non-zero elements along each row at appropriate distorted due to interpolation. We consider up sampled LR locations. We estimate this decimation matrix from the initial and variance ���������������� . It is of the size, M2× 1. The multivariate training images to find the best matching DCT coefficients for estimate. The procedure for estimating the decimation matrix 2 each of the blocks in the test image. is described below. n is the i.i.d noise vector with zero mean 2 Let CT (i , j), 1≤ ( i , j) ≤ = 4, be the DCT coefficient at noise probability density is given by 2������������������������ . location (i, j) in a 4 × 4 block of the test image. Similarly, let C(m)LR ( i , j) and C(m)HR (I , j), m = 1,2,……,L, be the DCT Our problem is to estimate z given y, which is an ill-posed coefficients at location( i, j) ,in the block at the same position inverse problem. It may be mentioned here that the in the mth up-sampled LR image and mth HR image. Here L observation captured is not blurred. In other words, we assume 348 http://sites.google.com/site/ijcsis/ ISSN 1947-5500 (IJCSIS) International Journal of Computer Science and Information Security, Vol. 8, No. 2, 2010 identity matrix for blur. Generally, the decimation model to A. Filter Bank obtain the aliased pixel intensities from the high resolution The Gaussian filters play an important role due to its nice low 1 1 …1 0 pixels, for a decimation factor of q, has the form [12] pass frequency property. The two dimensional Gaussian 2� 1 1… 1 � 1 �������� −�������� 0 �������� −�������� 0 −� + � �������� function can be defined as 1 2�������� 2 0 1 1 … .1 2�������� 2 �������� 2�������������������������������� �������� D= (4) G(x, y\x0,y0,σx,σy)= (5) Here (x0, y0) are location parameters and (σx σy) are scale The decimation matrix in Eq. (4) indicates that a low parameters. The Laplacian of Gaussian (LoG) filter is a resolution pixel intensity Y (i, j) is obtained by averaging the radially symmetric centers around Gaussian filter with (x0, y0) intensities of q2 pixels corresponding to the same scene in the = (0, 0) and σx = σy = T. Hence LoG filter can be represented high resolution image and adding noise intensity n (i, j). �������� 2 +�������� 2 − by F(x, y\0 ,0, T) = c.(x2 +y2 –T2)�������� �������� 2 (6) 1 Here c is a constant and T scale parameter. We can choose √2 different scales with T = , 1, 2,3, and so on.The Gabor filter with sinusoidal frequency ‘ω ‘and amplitude modulated by the Gaussian function can be represented by Training Set-1 Fw(x,y) = G(x,y\0,0,σx , σy)�������� −������������������������ (7) A simple case of Eq. (7) with both sine and cosine components G(x,y\0, 0,T, ��������) = c�������� 2�������� 2 ( 4(���������������������������������������� + ����������������������������������������)2 + 1 can chosen as • • (−����������������������������������������+����������������������������������������)2 (8) • By varying frequency and rotating the filter in - y plane, we as �������� = 00 ,300,600,900 and so on. can obtain a bank of filters. We can choose different scales T=2, 4, 6, 8 and so on. Similarly, the orientation can be varied B. Marginal Distribution Prior As mentioned earlier, the histograms of the filtered images estimate the marginal distribution of the image. We use this marginal distribution as a prior. We obtain the close Training set L approximation ZC of the HR image using discrete cosine transform based learning approach as described in section II Figure-1 and assume that the marginal distribution of the super-resolved obtain filtered images), where ���������������� , where α = 1,. . . . . . ,|��������|. image should match that of the close approximation ZC. Let B �������� IV TEXTURE MODELLING be a bank of filters. We apply each of the filters in B to ZC and We compute histogram �������� �������� of ���������������� . Similarly, we apply Natural images consist of smooth regions, edges and texture (��������) (��������) filtered images �������� �������� ,where �������� = 1, 2, 3…. |��������|. We compute areas. We regularize the solution using the texture preserving histogram ���������������� of ���������������� . We define the marginal distribution each of the filter in B to the initial HR estimate and obtain �������� �������� prior. We capture the features of texture by applying different filters to the image and compute histograms of the filtered images. These histograms estimate the marginal distribution of prior term as, CH =���������=1|HC − H α | |��������| the image. These histograms are used as the features of the image. We use a filter bank that consists of two kinds of α filters: Laplacian of Gaussian (LoG) filters and Gabor filters. (9) 349 http://sites.google.com/site/ijcsis/ ISSN 1947-5500 ̂ where f (i, j)is the original high resolution image and �������� (i , j) is (IJCSIS) International Journal of Computer Science and Information Security, Vol. 8, No. 2, 2010 estimated super-resolution image. V. SUPER-RESOLVING THE IMAGE The final cost function consisting of the data fitting term and marginal distribution prior term can be expressed as ̂ ��������= argument min� ‖��������−����������������‖ + �������� ∑��������=1|HC − H α |�. 2 |��������| 2 2���������������� α (10) Where, λ is a suitable weight for the regularization term. The cost function consists of non-linear term it cannot be (a) HR Image (b) Learnt Image minimized using simple gradient descent optimization technique. We employ particle swarm optimization and avoid the computationally complex optimization methods like simulated annealing. Let S be the swarm. The swarm S is populated of images Zp, p =1……|��������| expanded using existing interpolation techniques such as bi-cubic interpolation, lanczose interpolation and learning based approaches. Each pixel in this swarm is a particle. The dimension of the search space for each image is D = N × N. The i-th image of the swarm can be represented by (c) PSO Optimized Image a D-dimensional vector, Z =(Zi1, Zi2,…, ZiD)T . The velocity of particles in this image can be represented by another D- Figure-2 dimensional vector Vi = (vi1; vi2,….., viD)T. The best previously visited position of the i-th image is denoted as Pi = (pi1, pi2;…., piD)T .Defining ’ g ‘ as the index of the best ������������������������+1 = w������������������������ + c1 r1( ������������������������ - ������������������������ )+ c2r2 ( ������������������������ - ������������������������ ) particle in the swarm, the swarm is manipulated according to �������� �������� �������� �������� �������� �������� the following two equations [13], (11) ������������������������+1 = ������������������������ + ������������������������ (a) HR Image (b) Learnt Image �������� �������� �������� (12) where d =1, 2,….,D; i =1, 2,.….F; w is weighting function, r1 and r2 are random numbers uniformly distributed V1,V2,…are the iteration numbers, C1,C2,.. are cognitive and social parameter, respectively. The fitness function in our case is the cost function that has to be minimized. (c)PSO Optimized Image VI. EXPERIMENTAL RESULTS Figure-3 In this section, we present the results ( shown in the fig.2, fig.3 TABLE-1 and table-1) of the proposed method for the super-resolution. We compare the performance of the proposed method on the Image MMSE between MMSE between HR and PSO basis of quality of images. All the experiments were conducted Num HR and Learnt images images on real images. Each observed image is of size 128 ×128 pixels. The super-resolved images are also of size 128 × 128. 1 0.02173679178 0.02154759509 We used the quantitative measure Mean Square Error (MSE) � 2 ∑��������,�������� ���������(��������,�������� )−��������(��������,�������� )� for comparison of the results. The MSE used here is 2 0.01117524672 0.01107802761 ∑��������,�������� |��������(��������,�������� )|2 M.S.E = 350 http://sites.google.com/site/ijcsis/ ISSN 1947-5500 (IJCSIS) International Journal of Computer Science and Information Security, Vol. 8, No. 2, 2010 VII.CONCLUSION. [6] W.Freeman, T.Jones, E.Pasztor, “Example based super-resolution,” IEEE Computer GRAPHICS AND asPPLICATIONS, Vol.22,no.2,pp.56-65,2002. We have presented a technique to obtain super-resolution for [7]F.Brandi, R.de Queiroz, and D.Mukerjee, “super-resolution of video using an image captured using a low cost camera. The high key frames,”IEEE International Symposium on circuits and systems,pp.1608- frequency content of the super-resolved image is learnt from a 1611,2008. database of low resolution images and their high resolution versions. The suggested technique for learning the high [8]Y.W.Tai, W.S.tongand C.K.Tang,”Perceptually inspired and edge-directed color image super-resolution,”IEEE Computer Society Conference on frequency content of the super-resolved image yields close Computer Vision and Pattern recognition, vol.2,pp.1948-1955,2006. approximation to the solution. The LR observation is represented using linear model and marginal distribution is [9] T.Chan and J.Zhang,”An improved super-resolution with manifold used as prior information for regularization. The cost function learning and histogram matching,”Proc.IAPRInternational Conference on consisting of a data fitting term and a marginal distribution Biometric,pp.756-762,2006. prior term is optimized using particle swarm optimization. The optimization process converges rapidly. It may be [10] T.Chan,J.Zhang, J.Pu, and H.Huang,”Neighbot Embedding based super- concluded that the proposed method yields better results resolution algorithm through edge detection and feature selection,”Pattern Recognition Letters, Vol.30,no.5,pp,494-502,2009. considering both smoother regions as well as texture regions and greatly reduces the optimization time. 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[4] S.Chaudhuri, Super-resolution imaging, S.Chaudhuri, Ed.kluwer, 2001. [5] C.V.Jiji, M.V.Joshi, and S. Chaudhuri, ”Single frame image super- resolution using learned wavelet coefficients. “International Journal of Imaging Systems and Technology, Vol.14,no.3,pp.105-112,2004. AUTHORS PROFILE 351 http://sites.google.com/site/ijcsis/ ISSN 1947-5500