pnmrotate(1)

NAME

   pnmrotate - rotate a portable anymap by some angle

SYNOPSIS

   pnmrotate [-noantialias] angle [pnmfile]

DESCRIPTION

   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.

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.

                            12 January 1991                   pnmrotate(1)

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Free and Open Source Software