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.

   At the edges of the convolved image, where the convolution matrix would
   extend over the edge of the image,  pnmconvol  just  copies  the  input
   pixels 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

                           26 November 1994                   pnmconvol(1)

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