rotate a portable anymap by some angle
see also :
pnmshear - pnmflip - ppmquant
[-noantialias] angle [pnmfile]
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portable anymap as input. Rotates it by the specified angle
and produces a portable anymap 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),
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 to do a 90 degree rotation and then
pnmrotate less than 45 degrees back the other
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. 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.
All flags can
be abbreviated to their shortest unique prefix.
"A Fast Algorithm for General Raster Rotation" by Alan Paeth,
Graphics Interface ’86, pp. 77-81.
pnmflip , pnm, ppmquant
1989, 1991 by Jef Poskanzer.