pnmrotate (1)
rotate a portable anymap by some angle
SYNOPSIS
pnmrotate
[ -noantialias ]
angle
[ pnmfile ]
DESCRIPTION
Reads a portable anymap as input.
Rotates it by the specified angle
and produces a portable anymap as output.
rotation
If the input file is in color, the output will be too,
otherwise it will be grayscale.
The angle is in degrees (floating point), measured counter-clockwise.
It can be negative, but it should be between -90 and 90.
Also, for rotations greater than 45 degrees you may get better results
if you first use
pnmflip
pnmflip
to do a 90 degree rotation and then
pnmrotate
less than 45 degrees back the other direction
The rotation algorithm is Alan Paeth's three-shear method.
Each shear is implemented by looping over the source pixels and distributing
fractions to each of the destination pixels.
This has an "anti-aliasing" effect - it avoids jagged edges and similar
artifacts.
anti-aliasing
However, it also means that the original colors or gray levels in the image
are modified.
If you need to keep precisely the same set of colors, you can use the
-noantialias
flag. This does the shearing by moving pixels without changing their values.
If you want anti-aliasing and don't care about the precise colors, but
still need a limited *number* of colors, you can run the result through
ppmquant .
ppmquant
All flags can be abbreviated to their shortest unique prefix.
REFERENCES
"A Fast Algorithm for General Raster Rotation" by Alan Paeth,
Graphics Interface '86, pp. 77-81.
SEE ALSO
AUTHOR
Copyright (C) 1989, 1991 by Jef Poskanzer.
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