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Pnmshear User Manual(0) Pnmshear User Manual(0)
NAME
pnmshear - shear a PNM image by a specified angle
SYNOPSIS
pnmshear
[-noantialias] [-background=color] angle [pnmfile]
All options can be abbreviated to their shortest unique prefix. You
may use two hyphens instead of one to designate an option. You may use
either white space or equals signs between an option name and its
value.
DESCRIPTION
This program is part of Netpbm(1)
pnmshear reads a PNM image as input and shears it by the specified
angle and produce a PNM image as output. If the input file is in
color, the output will be too, otherwise it will be grayscale. The
angle is in degrees (floating point), and measures this:
+-------+ +-------+
| | |\ \
| OLD | | \ NEW \
| | |an\ \
+-------+ |gle+-------+
If the angle is negative, it shears the other way:
+-------+ |-an+-------+
| | |gl/ /
| OLD | |e/ NEW /
| | |/ /
+-------+ +-------+
The angle should not get too close to 90 or -90, or the resulting image
will be unreasonably wide.
pnmshear does the shearing by looping over the source pixels and dis-
tributing fractions to each of the destination pixels. This has an
'anti-aliasing' effect - it avoids jagged edges and similar artifacts.
However, it also means that the original colors in the image are modi-
fied and there are typically more of them than you started with. If
you need to keep precisely the same set of colors, see the -noantialias
option. If the expanded palette is a problem, you can run the result
through pnmquant.
OPTIONS
-background=color
This determines the color of the background on which the sheared
image sits.
Specify the color (color) as described for the argument of the
ppm_parsecolor() library routine <libppm.html#colorname> .
By default, if you don't specify this option, pnmshear selects
what appears to it to be the background color of the original
image. It determines this color rather simplistically, by tak-
ing an average of the colors of the two top corners of the
image.
This option was new in Netpbm 10.37 (December 2006). Before
that, pnmshear always behaved as is the default now.
-noantialias
This option forces pnmrotate to simply move pixels around
instead of synthesizing output pixels from multiple input pix-
els. The latter could cause the output to contain colors that
are not in the input, which may not be desirable. It also prob-
ably makes the output contain a large number of colors. If you
need a small number of colors, but it doesn't matter if they are
the exact ones from the input, consider using pnmquant on the
output instead of using -noantialias.
Note that to ensure the output does not contain colors that are
not in the input, you also must consider the background color.
See the -background option.
SEE ALSO
pnmrotate(1) , pamflip(1) , pnmquant(1) , pnm(5)
AUTHOR
Copyright (C) 1989, 1991 by Jef Poskanzer.
netpbm documentation 27 November 2006 Pnmshear User Manual(0)
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