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pnmrotate(1) Unix Programmer's Manual pnmrotate(1)
NNNAAAMMMEEE
pnmrotate - rotate a portable anymap by some angle
SSSYYYNNNOOOPPPSSSIIISSS
pppnnnmmmrrroootttaaattteee [-nnnoooaaannntttiiiaaallliiiaaasss] _a_n_g_l_e [_p_n_m_f_i_l_e]
DDDEEESSSCCCRRRIIIPPPTTTIIIOOONNN
Reads a 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
_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 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
---nnnoooaaannntttiiiaaallliiiaaasss 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 prefix.
RRREEEFFFEEERRREEENNNCCCEEESSS
"A Fast Algorithm for General Raster Rotation" by Alan Paeth, Graphics
Interface '86, pp. 77-81.
SSSEEEEEE AAALLLSSSOOO
pnmshear(1), pnmflip(1), pnm(5), ppmquant(1)
AAAUUUTTTHHHOOORRR
Copyright (C) 1989, 1991 by Jef Poskanzer.
12 January 1991 1