pnmconvol(1)            UNIX System V(13 January 1991)             pnmconvol(1)


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.












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