What is SSIM's output range?

[ljeonjko]

Hi!
I'm using your implementation of SSIM as a loss function. I need to know the output range: is it [0,1] or [-1,1]?
Cheers!

[ljeonjko]

Found it in previous discussions. It's [0,1], meaning the loss function can be implemented as 1 - ssim(x,y).

[francois-rozet]

Hello @ljeonjko 👋,

I had to think about it, but the output range of SSIM is [-1, 1]. Indeed, for the first term, if x and y are positives, the range is [0, 1], but the contrast sensitivity (CS) is some kind of correlation between x and y, which means it ranges from -1 to 1.

However, this does not change anything if you want a loss function. Any function of the type a - b * ssim(x, y), with b strictly positive, would have gradients in the correct direction.

P.S. I could not find the previous discussion where this was mentioned. However, it is written in the README that the output range is [0, 1] which I will fix shortly.