Finding correlation between two images

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Mohammad Saquib Khan 2021-9-24

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https://ww2.mathworks.cn/matlabcentral/answers/1459919-finding-correlation-between-two-images

评论： Image Analyst 2021-9-25

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?

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Bjorn Gustavsson 2021-9-24

Have you looked in the documentation how normxcorr2 normalizes the x-correlation values for the different shifts, it seems a "natural" guess that it properly scales with the number of samples, while imfilter might "not bother with that" since its purpose is mainly to filter images.

Mohammad Saquib Khan 2021-9-24

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?

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Answer 1

Image Analyst 2021-9-24

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链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1459919-finding-correlation-between-two-images#answer_794289

normxcorr2_demo.m

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.

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Mohammad Saquib Khan 2021-9-24

If I use xcorr2, I just get a simple white image. But when I used normxcorr2, I get an image where I can see a pattern. So I guess normxcorr2 is the right way. Thank you so much for your help!

Image Analyst 2021-9-25

在 MATLAB Online 中打开

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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