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 convo-
    lution 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.

    At the edges of the convolved image, where the convolution matrix would
    extend over the edge of the image, pnmconvol just copies the input pix-
    els directly to the output.

SEE ALSO
   pnmsmooth(1), pnm(5)

AUTHORS
    Copyright (C) 1989, 1991 by Jef Poskanzer.
    Modified 26 November 1994 by Mike Burns, burns@chem.psu.edu