negative value structural similarity index

Hello everyone ,

I aim to compare two 2D arrays which represent the reflectivity of an object.

I generate with the inbuilt ssim function the map of the structural similiraty index. And in some regions i get a negative value. I have read the description of the index and i do not get the physical meaning of a negative value.

P.S : my two images are positive double real numbers

[Jonas]

have a look here for negative values:

https://de.mathworks.com/matlabcentral/answers/1948883-what-is-the-possible-output-range-of-ssim

[Albert Hall]

Thx for your answer Jonas but this does not help. I have two positive real matrixes which values are between 0 and 1 . And I have some local negative ssim values (not especially -1 ,-0.2 for instance) . I don't get the physical meaning . My images are not anti-correlated.

Answer 1

[Matt J]

A negative SSIM value means the values in those regions tend to be negatively correlated. When X goes low, then Y tends to go high.

[Albert Hall]

okay , I got your point , thanks

[Matt J]

You're quite welcome, but if your question is resolved, please Accept-click the answer.