How to calculate PSNR of compressed images, and... Learn more about how to calculate psnr of a compressed image, psnr, image compression Image Processing Toolbox
You can get a mask of the tank by thresholding at some value, like 40 or 100 or whatever works well
No, only if they're floating point images, not integer images like uint8. You need to cast them to double so that your numbers don't get clipped at 0 if they would go negative.