You can set the threshold for whatever you want. I have no idea how you'd even use it. I'm not sure what you did. You cannot sensibly use ssim to compare any of those three images to each other - it just doesn't make sense. Just think about it. They're far too different. So setting a threshold for it would also not make sense. At best you might judge how good a job your segmenter did by comparing your ground truth to the "new" image, but even that is not good. That's because your "new Image" doesn't look like your ground truth image. It looks like it's been skeletonized so you'd have to use something like dice to compare the skeleton of your ground truth image to the "new" skeletonized image. Or else you can use dice to compare a binarized version of the ground truth image to the "new" image before you binarized it.
Again, ssim would not be used here. It's meant for comparing two similar images, such as a gray scale image and one that underwent a round trip compression and decompression for example.
