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)