pnmconvol(1)pnmconvol(1)NAMEpnmconvol - general MxN convolution on a portable anymap
SYNOPSISpnmconvol 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 ALSOpnmsmooth(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)