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pnmconvol
!!!pnmconvol
NAME
SYNOPSIS
DESCRIPTION
SEE ALSO
AUTHORS
----
!!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.


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.
!!SEE ALSO


pnmsmooth(1), pnm(5)
!!AUTHORS


Copyright (C) 1989, 1991 by Jef Poskanzer.
Modified 26 November 1994 by Mike Burns,
burns@chem.psu.edu
----

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Last edited on Tuesday, June 4, 2002 12:22:43 am by "perry"

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