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 rela-
tive.
Here is a sample convolution file; it does a simple aver-
age 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 normaliza-
tion 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 ALSOpnmsmooth(1), pnm(5)AUTHOR
Copyright (C) 1989, 1991 by Jef Poskanzer.
13 January 1991 1
