# Finding correlation between two images

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Mohammad Saquib Khan on 24 Sep 2021
Commented: Image Analyst on 25 Sep 2021
Suppose I want to perform correlation between two images. I know there is a function normxcorr2 which can be used to find the correlation between two images (img1, img2) like this
When I run the imfilter command and use the imshow() command to output the resulting image, I can see that it is different from what I get when I output the resulting image using normxcorr2 command.The image generated through normxcorr2 looks more like what i would expect from the correlation of the two images. What's the difference between these two methods?
C = normxcorr2(img1, img2)
But can I find the correlation using imfilter? Does this command perform correlation?
corr = imfilter(img1, img2, 'corr','replicate');
When I run the imfilter command and use the imshow() command to output the resulting image, I can see that it is different from what I get when I output the resulting image using normxcorr2 command.
imshow(C);
figure
imshow(corr);
The image generated through normxcorr2 looks more like what i would expect from the correlation of the two images. What's the difference between these two methods? Is using imfilter even logical?
##### 2 CommentsShowHide 1 older comment
Mohammad Saquib Khan on 24 Sep 2021
I haven't yet looked into the documentation but I will surely do that now. What if after computing my variable corr, I normalise it? Will then the the two resulting images be same?

Image Analyst on 24 Sep 2021
normxcorr2() scans the image with a window and does a normalization at each location. imfilter() does not. It simply moves the window, multiplies the pixels together, and sums -- no normalization. You cannot normalize a xcorr2() or imfilter() output image to make it look like what it would look like if it had normalized the subimages at each location.
Image Analyst on 25 Sep 2021
If your output is a floating point image you're going to have to make sure it's in the range 0-1 or else any values more than 1 will show up as white. Try casting to uint8 before display. If it's a gray scale floating point image, try
imshow(yourImage, []);

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