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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  compres-
       sion 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 dif-
       ference 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 computa-
       tion, 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)