Pnmconvol User Manual(0) Pnmconvol User Manual(0)NAMEpnmconvol - general MxN convolution on a PNM image
SYNOPSISpnmconvol
convolution_matrix_file [-nooffset] [pnmfile]
Minimum unique abbreviation of option is acceptable. You may use dou‐
ble hyphens instead of single hyphen to denote options. You may use
white space in place of the equals sign to separate an option name from
its value.
DESCRIPTION
This program is part of Netpbm(1).
pnmconvol reads two PNM images as input, convolves the second using the
first, and writes a PNM image 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 (sometimes called a convolution kernel), which
you supply by way of the PNM image in the file you identify with the
convolution_matrix_file argument. There are two ways pnmconvol inter‐
prets the PNM convolution matrix image pixels as weights: with offsets,
and without offsets.
The simpler of the two is without offsets. That is what happens when
you specify the -nooffset option. In that case, pnmconvol simply nor‐
malizes the sample values in the PNM image by dividing by the maxval.
For example, here is a sample convolution file that causes an output
pixel to be a simple average of its corresponding input pixel and its 8
neighbors, resulting in a smoothed image:
P2
3 3
18
2 2 2
2 2 2
2 2 2
(Note that the above text is an actual PGM file -- you can cut and
paste it. If you're not familiar with the plain PGM format, see theP‐
GMformatspecification(1)).
pnmconvol divides each of the sample values (2) by the maxval (18) so
the weight of each of the 9 input pixels gets is 1/9, which is exactly
what you want to keep the overall brightness of the image the same.
pnmconvol creates an output pixel by multiplying the values of each of
9 pixels by 1/9 and adding.
Note that with maxval 18, the range of possible values is 0 to 18.
After scaling, the range is 0 to 1.
For a normal convolution, where you're neither adding nor subtracting
total value from the image, but merely moving it around, you'll want to
make sure that all the scaled values in (each plane of) your convolu‐
tion PNM add up to 1, which means all the actual sample values add up
to the maxval.
When you don't specify -nooffset, pnmconvol applies an offset, the pur‐
pose of which is to allow you to indicate negative weights even though
PNM sample values are never negative. In this case, pnmconvol sub‐
tracts half the maxval from each sample and then normalizes by dividing
by half the maxval. So to get the same result as we did above with
-nooffset, the convolution matrix PNM image would have to look like
this:
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 normalization step divides by 18/2 = 9, which makes it 1/9 -
exactly what you want. The equivalent matrix for 5x5 smoothing would
have maxval 50 and be filled with 26.
Note that with maxval 18, the range of possible values is 0 to 18.
After offset, that's -9 to 9, and after normalizing, the range is -1 to
1.
For a normal convolution, where you're neither adding nor subtracting
total value from the image, but merely moving it around, you'll want to
make sure that all the offset, scaled values in (each plane of) your
convolution PNM add up to 1. That means the actual sample values, less
half the maxval, add up to half the maxval as in the example above.
The convolution file will usually be a PGM, so that the same convolu‐
tion gets applied to each color component. However, if you want to use
a PPM and do a different convolution to different colors, you can cer‐
tainly 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 pix‐
els directly to the output.
The convolution computation can result in a value which is outside the
range representable in the output. When that happens, pnmconvol just
clips the output, which means brightness is not conserved.
HISTORY
The -nooffset option was new in Netpbm 10.23 (July 2004).
SEE ALSOpnmsmooth(1), pgmmorphconv(1), pnmnlfilt(1), pgmkernel(1), pamgauss(1),
pnm(1)AUTHORS
Copyright (C) 1989, 1991 by Jef Poskanzer. Modified 26 November 1994
by Mike Burns, burns@chem.psu.edu
netpbm documentation 29 June 2005 Pnmconvol User Manual(0)
