shear a portable anymap by some angle
see also :
pnmrotate - pnmflip - ppmquant
[-noantialias] angle [pnmfile]
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portable anymap as input. Shears 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), and
| | |\ \
| OLD | | \ NEW \
| | |an\ \
If the angle is negative, it shears the other way:
| | |gl/ /
| OLD | |e/ NEW /
| | |/ /
The angle should not get too close to 90 or -90, or the
resulting anymap will be unreasonably wide.
The shearing 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.
pnmflip , pnm, ppmquant
1989, 1991 by Jef Poskanzer.