pnmconvol(1)                                                  General Commands Manual                                                 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 off-
       set 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)