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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 neighbors, 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 convolution 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, email@example.com
26 November 1994 pnmconvol(1)