pnmpsnr


SYNOPSIS
       pnmpsnr [pnmfile1] [pnmfile2]


DESCRIPTION
       Reads two PBM, PGM, or PPM files, or PAM equivalents, as input.  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 betwee 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.


SEE ALSO
       pnm(5)



                                 04 March 2001                      pnmpsnr(1)
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