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pnmrotate.1
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pnmrotate(1) USER COMMANDS pnmrotate(1)
NAME
pnmrotate - rotate a portable anymap by some angle
SYNOPSIS
pnmrotate [-noantialias] _✓a_✓n_✓g_✓l_✓e [_✓p_✓n_✓m_✓f_✓i_✓l_✓e]
DESCRIPTION
Reads a portable anymap as input. Rotates it by the speci-
fied 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 _✓p_✓n_✓m_✓f_✓l_✓i_✓p to do a 90 degree rotation and then _✓p_✓n_✓m_✓r_✓o_✓-
_✓t_✓a_✓t_✓e 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 pix-
els. 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 modi-
fied. 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 _✓p_✓p_✓m_✓q_✓u_✓a_✓n_✓t.
All flags can be abbreviated to their shortest unique pre-
fix.
REFERENCES
"A Fast Algorithm for General Raster Rotation" by Alan
Paeth, Graphics Interface '86, pp. 77-81.
SEE ALSO
pnmshear(1), pnmflip(1), pnm(5), ppmquant(1)
AUTHOR
Copyright (C) 1989, 1991 by Jef Poskanzer.
Sun Release 4.0 Last change: 12 January 1991 1