Implementation of metrics

[ziqipang]

Thank you for your excellent work! Your idea is innovative to me, and your code is well-structured. However, on reviewing your code, I have some suggestions that potentially improve your performance.

Your implementation of ssim and psnr are from open-source codes on GitHub. However, I highly recommend you consider choosing the standard implementation in the package skimage. In my research project, I once use the same code as you are now. But after I switch to skimage for evaluation, the performance experiences an improvement. Although I do not know the reason for this, I suggest you try using metrics in skimage, which may reveal a better performance.