fr_psnr

代码说明：

  PSNR is used to measure the difference between two images. It is defined as PSNR = 20 * log10(b/rms) where b is the largest possible value of the signal (typically 255 or 1), and rms is the root mean square difference between two images. The PSNR is given in decibel units (dB), which measure the ratio of the peak signal and the difference between two images. An increase of 20 dB corresponds to a ten-fold decrease in the rms difference between two images.