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pnmconvol - general MxN convolution on a portable anymap
pnmconvol convolutionfile [pnmfile]
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 neigh-
bors, resulting in a smoothed image:
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 convolu-
tion to different colors, you can certainly do that.
At the edges of the convolved image, where the convolution matrix would extend over the
edge of the image, pnmconvol just copies the input pixels directly to the output.
Copyright (C) 1989, 1991 by Jef Poskanzer.
Modified 26 November 1994 by Mike Burns, firstname.lastname@example.org
26 November 1994 pnmconvol(1)
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