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1993-01-08
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pnmconvol(1) Unix Programmer's 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 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
pnmsmooth(1), pnm(5)
AUTHOR
Copyright (C) 1989, 1991 by Jef Poskanzer.
13 January 1991 1