SSIM are negative

[icey-zhang]

Hello, what's wrong with code that comes out with negative values？

[nickdalfarra]

I'm not 100% sure, but there seem to be different conventions on the range of SSIM implementations. Some output to [0,1], others to [-1,1]. I don't have an example of two images producing a perfectly bad SSIM of -1, but you could always just linearly transform the SSIM output via (SSIM + 1)/2 to make the range [0,1].

[TalTaiber]

Hello, what's wrong with code that comes out with negative values？

Just a guess, since the author did not repond:
Higher SSIM corresponds to greater similarity between the two images. However, notice that in the author's explanation we have:
ssim_loss = pytorch_ssim.SSIM()
Since the optimizer tries to minimize the loss, we have to define the SSIM loss as negative SSIM, so that when the loss is low the SSIM will be high.