Reproducability with scikit-image ssim

[MortenHannemose]

It would be great if there was a flag to make the results identical to the output of compare_ssim from scikit image.
Their implementation uses no padding, but there's still a slight difference in my results, that I haven't found.

[veritas9872]

Hello. I would also like it if there were a comparison with skimage.compare_ssim. Could you include a unit test which asserts that the outputs are at least np.allclose with the outputs from skimage?

[ucalyptus]

Nice issue

[tmabraham]

The difference may be from the constants C1=(0.01)**2 and C2=(0.03)**2 which assume the range of pixel values is 0 to 1. If instead it is C1=(0.01*255)**2 and C2=(0.03*255)**2, along which removing padding, then the implementations match.