This program is part of Netpbm.
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
anymap will be unreasonably wide.
pnmshear does the shearing 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 option. 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 pnmquant.
All options can be abbreviated to their shortest unique prefix.