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)