This program is part of Netpbm.

pnmpsnr reads two PBM, PGM, or PPM files, or PAM equivalents, as input
and prints the peak signal-to-noise ratio (PSNR) difference between
the two images. This metric is typically used in image compression
papers to rate the distortion between original and decoded image.

If the inputs are PBM or PGM, pnmpsnr prints the PSNR of the luminance
only. Otherwise, it prints the separate PSNRs of the luminance, and
chrominance (Cb and Cr) components of the colors.

The PSNR of a given component is the ratio of the mean square differ-
ence of the component for the two images to the maximum mean square
difference that can exist between any two images. It is expressed as
a decibel value.

The mean square difference of a component for two images is the mean
square difference of the component value, comparing each pixel with
the pixel in the same position of the other image. For the purposes
of this computation, components are normalized to the scale [0..1].

The maximum mean square difference is identically 1.

So the higher the PSNR, the closer the images are. A luminance PSNR
of 20 means the mean square difference of the luminances of the pixels
is 100 times less than the maximum possible difference, i.e. 0.01.


netpbm documentation 04 March 2001 Pnmpsnr User Manual(0)