i have a directory of 2500 color images. i want to compare each image with others to know is there in directory similar image with this?
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Well it seems someone's allready done the job for you: http://www.mathworks.se/matlabcentral/fileexchange/42008-content-based-image-retrieval
One way to compare the images quickly would be compute a structural quality metric such as SSIM between the two images. It is a popular metric and a quick web search can get you all the necessary info.
It is easy to compute and does more than correlation. But you would need to resize one of the images to match the size of the other. You can use IMRESIZE from the Image Processing Toolbox for that. Although IMSHOWPAIR can be helpful if you want to visualize the mismatch between the two images, it won't give you a metric on the amount of mismatch.