PSNR
From Avisynth
PSNR is measured in decibels. It's defined as
with
where M = width
height is the number of pixels in a frame and width, height are the width and height of the frame, respectively, in pixels.
The double-sum term states that (j,k) runs over all pixels of the frame, summing the absolute value of the difference of I, K functions raised to the power of 2.
For more details see: http://en.wikipedia.org/wiki/PSNR .

