PNMCONVOL

NAME

      pnmconvol - general MxN convolution on a portable anymap

SYNOPSIS

      pnmconvol convolutionfile [pnmfile]

DESCRIPTION

      Reads two portable anymaps as input.  Convolves the second using the
      first, and writes a portable anymap as output.
      Convolution means replacing each pixel with a weighted average of the
      nearby pixels.  The weights and the area to average are determined by
      the convolution matrix.  The unsigned numbers in the convolution file
      are offset by -maxval/2 to make signed numbers, and then normalized,
      so the actual values in the convolution file are only relative.
      Here is a sample convolution file; it does a simple average of the
      nine immediate neighbors, resulting in a smoothed image:
          P2
          3 3
          18
          10 10 10
          10 10 10
          10 10 10
      To see how this works, do the above-mentioned offset: 10 - 18/2 gives
      1.  The possible range of values is from 0 to 18, and after the offset
      that's -9 to 9.  The normalization step makes the range -1 to 1, and
      the values get scaled correspondingly so they become 1/9 - exactly
      what you want.  The equivalent matrix for 5x5 smoothing would have
      maxval 50 and be filled with 26.
      The convolution file will usually be a graymap, so that the same
      convolution gets applied to each color component.  However, if you
      want to use a pixmap and do a different convolution to different
      colors, you can certainly do that.

SEE ALSO

      pnmsmooth(1), pnm(5)

AUTHOR

      Copyright (C) 1989, 1991 by Jef Poskanzer.