.TH pnmconvol 1 "13 January 1991"
.IX pnmconvol
.SH NAME
pnmconvol - general MxN convolution on a portable anymap
.SH SYNOPSIS
.B pnmconvol
.I convolutionfile
.RI [ pnmfile ]
.SH DESCRIPTION
Reads two portable anymaps as input.
Convolves the second using the first,
and writes a portable anymap as output.
.IX convolution
.PP
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.
.PP
Here is a sample convolution file;
it does a simple average of the nine immediate neighbors, resulting
in a smoothed image:
.nf
    P2
    3 3
    18
    10 10 10
    10 10 10
    10 10 10
.fi
.PP
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.
.PP
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.
.SH "SEE ALSO"
pnmsmooth(1), pnm(5)
.SH AUTHOR
Copyright (C) 1989, 1991 by Jef Poskanzer.
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