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 spec-
       ified angle and produces a portable anymap as output.   If
       the input file is in color, the output will be too, other-
       wise 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  rota-
       tion  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 pix-
       els 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  val-
       ues.   If  you want anti-aliasing and don't care about the
       precise colors, but still need a limited *number* of  col-
       ors, you can run the result through ppmquant.

       All flags can be abbreviated to their shortest unique pre-
       fix.

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.