pnmshear Man page

pnmshear General Commands Manual pnmshear


pnmshear – shear a portable anymap by some angle


pnmshear [-noantialias] angle [pnmfile]


Reads a 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 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
anymap will be unreasonably wide.

The shearing is implemented 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 or gray levels in the
image are modified. If you need to keep precisely the same set of col‐
ors, you can use the -noantialias flag. This does the shearing by mov‐
ing pixels without changing their values. If you want anti-aliasing
and don’t care about the precise colors, but still need a limited *num‐
ber* of colors, you can run the result through ppmquant.

All flags can be abbreviated to their shortest unique prefix.


pnmrotate, pnmflip, pnm(5), ppmquant


Copyright (C) 1989, 1991 by Jef Poskanzer.

12 January 1991 pnmshear