Why ssim and corr2 give totally different results

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Coryn Melissa LLamoza Carabali 2020-10-25

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https://ww2.mathworks.cn/matlabcentral/answers/625513-why-ssim-and-corr2-give-totally-different-results

回答： Raunak Gupta 2020-12-3

Hi, new to matlab and although I have read the definition of both functions. I still can't really understand why ssim always gives values close to 1, like all images are super similar. While it's clearly that the images are changing a little. corr2 seems to be more reliable, but need to explain the 'why'. Does somebody have an Idea?

Thank ypu very much in davance

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Mathieu NOE 2020-10-26

hello

maybe it would be helpful to share your code and explain what you are looking for

all the best

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Answer 1

Raunak Gupta 2020-12-3

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Hi,

The corr2 gives a simple correlation between two matrices which is highly dependent on the values present in both the matrices, even a slight noise can suggest that both the images are very different (and there will be a drop in peak value of correlation matrix).

Whereas ssim is much more extensive and robust due to different properties modeled for finding similarity between two images. You will see cross-correlation is one of them, but certainly other terms are also there in the formula mentioned here. That is why if there is a lot of change in the structure and luminance of the image then only the ssim score will be low. You may have a look at the reference mentioned here.