pnmconvol (1)
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
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.
SEE ALSO
AUTHORS
Copyright (C) 1989, 1991 by Jef Poskanzer.
Modified 26 November 1994 by Mike Burns, burns@chem.psu.edu
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