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


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






       This program is part of Netpbm(1).

       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)

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