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 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.

SEE ALSO

       pnmsmooth(1), pnm(5)

AUTHOR

       Copyright (C) 1989, 1991 by Jef Poskanzer.

                                13 January 1991                   pnmconvol(1)