Pnmpsnr User Manual(0)					Pnmpsnr User Manual(0)



NAME
       pnmpsnr - compute the difference between two images (the PSNR)


SYNOPSIS
       pnmpsnr

       [pnmfile1]

       [pnmfile2]


DESCRIPTION
       This program is part of Netpbm(1).

       pnmpsnr	reads two PBM, PGM, or PPM files, or PAM equivalents, as input
       and prints the magnitude of difference between the two images as a peak
       signal-to-noise	ratio  (PSNR)  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 maximum mean square
       difference of component values that could exist between the two	images
       (a  measure  of the information content in an image) to the actual mean
       square difference for the two subject 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.


SEE ALSO
       pnm(1)



netpbm documentation		 04 March 2001		Pnmpsnr User Manual(0)