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       pnmrotate - rotate a portable anymap by some angle


       pnmrotate [-noantialias] angle [pnmfile]


       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 pnmflip to
       do a 90 degree rotation and then pnmrotate 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
       -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.


       "A  Fast Algorithm for General Raster Rotation" by Alan Paeth, Graphics Interface '86, pp.


       pnmshear(1), pnmflip(1), pnm(5), ppmquant(1)


       Copyright (C) 1989, 1991 by Jef Poskanzer.

                                         12 January 1991                             pnmrotate(1)