pnmrotate(1)                General Commands Manual               pnmrotate(1)

       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 an-
       gle 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 pn-
       mflip  to  do  a 90 degree rotation and then pnmrotate less than 45 de-
       grees 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 frac-
       tions 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. 77-81.

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

       Copyright (C) 1989, 1991 by Jef Poskanzer.

                                12 January 1991                   pnmrotate(1)
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