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       pnmpsnr - compute the difference between two portable anymaps


       pnmpsnr [pnmfile1] [pnmfile2]


       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 difference 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

       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.



                                          04 March 2001                                pnmpsnr(1)