Image fusion is the hottest research subtopic of image processing. Fusion is basically extraction of best inputs and conveying to the output. For making an image, which is more suitable for segmentation, extraction of features, object recognition and human visual system, Image fusion is frequently used. It combines complimentary information from different image of the same scene in a single image. Image fusion can be performed at four different process levels which are signal, pixel, feature and decision level according to the stage at which levels fusion takes place. This paper provides survey of some of the various existing techniques applied for image fusion and comparative study of all techniques concludes better approach for its future research. In this paper we also try to find it out what are things that were neglected by the researchers, so our main objective was to find out the gaps in existing literature.

BWT, DCT, DWT, DT-CWT, Image fusion , PCA

[1].P.J. Burt, ―The pyramid as a structure for efficient computation, in: A. Rosenfeld (Ed.), Multiresolution Image Processing and Analysis‖, Springer-Verlag, Berlin, 1984, pp. 6�35.
[2].E.H. Adelson, ―Depth-of-Focus Imaging Process Method, United States‖ Patent 4,661,986 (1987).
[3].R.D. Lillquist, ―Composite visible/thermal-infrared imaging apparatus. United States‖ Patent 4,751,571 (1988).
[4].N. Nandhakumar, J.K. Aggarwal, ―Integrated analysis of thermal and visual images for scene interpretation‖, IEEE Transactions on Pattern Analysis and Machine Intelligence 10 (4) (1988) 469�481.
[5].A. Toet, ―Image fusion by a ratio of low-pass pyramid, Pattern Recognition Letters‖ (1989) 245�253.
[6].S.K. Rogers, C.W. Tong, M. Kabrisky, J.P. Mills, ―Multisensor fusion of ladar and passive infrared imagery for target segmentation‖, Optical Engineering 28 (8) (1989) 881�886.
[7].L.J. Chipman, Y.M. Orr, L.N. Graham, ―Wavelets and image fusion‖, in: Proceedings of the International Conference on Image Processing, Washington, USA, 1995, pp. 248�251
[8] .Koren, I., Laine, A., Taylor, F.,‖ Image fusion using steerable dyadic wavelet‖. In: Proceedings of the Internatinonal Conference. on Image Processing, Washington, USA, 1995. pp. 232�235.
[9].Neural processing of targets - Waxman, Seibert, et al. � 1995
[10].Li, Hui, B. S. Manjunath, and Sanjit K. Mitra. "Multisensor image fusion using the wavelet transforms." Graphical
models and image processing , vol. 3,pp. 235-245. IEEE, 1997.
[11].O.Rockinger. �Image sequence fusions using a shift-invariant wavelet transform.� In image processing , 1997 International Conference on, vol. 3, pp. 288-291. IEEE, 1997.
[12].Y-T. Kim. �Contrast enhancement using brightness preserving bihistogram equalisation.� In Consumer Electronics, 1997 International Conference on, vol. 43, pp. 1-8., IEEE, 1997
[13].Wang, Qiang, and Yi Shen. "The effects of fusion structures on image fusion performances." In Instrumentation and Measurement Technology Conference, 2004. IMTC 04. Proceedings of the 21st IEEE, vol. 1, pp. 468-471. IEEE, 2004.
[14].He, D-C., Li Wang, and Massalabi Amani. "A new technique for multi-resolution image fusion." In Geoscience and Remote Sensing Symposium, 2004. IGARSS`04. Proceedings. 2004 IEEE International, vol. 7, pp. 4901-4904.IEEE, 2004.
[15].T.Zaveri, M.Zaveri, V.Shah and N.Patel. �A Novel Region Based Multifocus Image Fusion Method.� In Digital Image Processing, 2009 International Conference on, pp. 50-54. IEEE, 2009
[16].Anjali Malviya, S. G. Bhirud .‖Image Fusion of Digital Images‖ International Journal of Recent Trends in Engineering, Vol 2, No. 3, November 2009
[17].Susmitha Vekkot and Pancham Shukla, A Novel Architecture for Wavelet based Image Fusion, Journal of World Academy of Science, Engineering and Technology, 2009. pp. 32-33.
[18]. Pei, Yijian, Huayu Zhou, Jiang Yu, and Guanghui Cai. "The improved wavelet transforms based image fusion algorithm and the quality assessment." In Image and Signal Processing (CISP), 2010 3rd International Congress on,
vol. 1, pp. 219-223. IEEE, 2010.
[19].Haghighat, Mohammad Bagher Akbari, Ali Aghagolzadeh, and Hadi Seyedarabi. "Real-time fusion of multi-focus images for visual sensor networks." In Machine Vision and Image Processing (MVIP), 2010 6th Iranian, pp. 1-6. IEEE, 2010.
[20].Lavanya, A., K. Vani, S. Sanjeevi, and R. S. Kumar, "Image fusion of the multi-sensor lunar image data using wavelet combined transformation." In Recent Trends in Information Technology (ICRTIT), 2011 International
Conference on, pp. 920-925. IEEE, 3-5 June, 2011
[21].Ghimire Deepak and Joonwhoan Lee. �Nonlinear Transfer Function-Based Local Approach for Color Image Enhancement.� In Consumer Electronics, 2011 International Conference on, pp. 858-865 . IEEE,2011
[22].Ren, Haozheng, Yihua Lan, and Yong Zhang. "Research of multi-focus image fusion based on M-band multi-wavelet transformation." In Advanced Computational Intelligence (IWACI), 2011 Fourth International Workshop on, pp. 395-398. IEEE, 2011
[23].Xing Su-xiaLian Xiao-fengChen Tian-hua, Xiao Hongbing.�Image Fusion Method Based on NSCT and Robustness Analysis.� In International Conference on Computer Distributed Control and Intelligent Environmental Monitoring, 2011,pp. 346-349. IEEE, 2011.
[24].Mohamed, M. A., and B. M. El-Den. "Implementation of image fusion techniques for multi-focus images using FPGA." In Radio Science Conference (NRSC), 2011 28th National, pp. 1-11. IEEE, 2011.
[25].Patil, Ujwala, and Uma Mudengudi. "Image fusion using hierarchical PCA." In image Information Processing (ICIIP), 2011 International Conference on, pp. 1-6. IEEE, 2011.
[26].A. Soma Sekhar, Dr.M.N.Giri Prasad. �A Novel Approach Of Image Fusion On MR And CT Images Using Wavelet Transforms.� IEEE Trans. on Image Proc., pp. 172-176. IEEE, 2011.
[27].M .Chandana,S. Amutha, and Naveen Kumar, ― A Hybrid Multi-focus Medical Image Fusion Based on Wavelet Transform‖. International Journal of Research and Reviews in Computer Science (IJRRCS) Vol. 2, No. 4, August 2011, ISSN: 2079-2557
[28].Chetan K. Solanki Narendra M. Patel, ―Pixel based and Wavelet based Image fusion Methods with their Comparative Study‖. National ConferenceonRecentTrendsinEngineeringandTechnology.13-14May2011
[29].KiranParmar, Rahul Kher. �A Comparative Analysis of Multimodality Medical Image Fusion Methods.� In Sixth Asia Modelling Symposium, 2012International Conference on, pp. 93-97. IEEE,2012.
[30].Aribi, Walid, Ali Khalfallah, Med Salim Bouhlel, and Noomene Elkadri. "Evaluation of image fusion techniques in nuclear medicine." In Sciences of Electronics, Technologies of Information and Telecommunications (SETIT), 2012 6th International Conference on, pp. 875-880. IEEE, 2012.
[31].Sruthy, S., Latha Parameswaran, and Ajeesh P. Sasi. "Image Fusion Technique using DT-CWT." In Automation, Computing, Communication, Control and Compressed Sensing (iMac4s), 2013 International Conference on, pp. 160-164. IEEE, 2013.
[32].Desale, Rajenda Pandit, and Sarita V. Verma. �Study and analysis of PCA, DCT & DWT based image fusion techniques." In Signal Processing Image Processing & Pattern Recognition (ICSIPR), 2013 International Conference on, pp. 66-69. IEEE, 2013.
[33].Shih-Gu Huang, �Wavelet for Image Fusion�.
[34].Om Prakash, RichaSrivastava, AshishKhare. �Biorthogonal Wavelet Transform Based Image Fusion Using Absolute Maximum Fusion Rule.� In Image processing, 2013 International Conference on Information and Communication Technologies, pp. 577-582.IEEE, 2013.
[35].Y.AsnathVictyPhamila, R.Amutha. �Discrete Cosine Transform based fusion of multi-focus images for visual sensor networks.� In Signal Processing, 2013 International Conference on, pp.161-170.IEEE, 2013.
[36] V.K Mishra, Shobhit kumar, �Design and implementation of image fusion system� In International journal of computer science and engineering . Vol.2, Issue-3, pp (182-186) March 2014, E-ISSN: 2347-269