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Pnmshear User Manual(0)                                Pnmshear User Manual(0)

       pnmshear - shear a PNM image by a specified angle


       [-noantialias] [-background=color] angle [pnmfile]

       All options can be abbreviated to their shortest unique prefix.  You may use two hyphens instead of one to des-
       ignate an option.  You may use either white space or equals signs between an option name and its value.

       This program is part of Netpbm(1).

       pnmshear reads a PNM image as input and shears it by the specified angle and produce a PNM image 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), and measures this:

           +-------+  +-------+
           |       |  |\       \
           |  OLD  |  | \  NEW  \
           |       |  |an\       \
           +-------+  |gle+-------+

       If the angle is negative, it shears the other way:
           +-------+  |-an+-------+
           |       |  |gl/       /
           |  OLD  |  |e/  NEW  /
           |       |  |/       /
           +-------+  +-------+

       The angle should not get too close to 90 or -90, or the resulting image will be unreasonably wide.

       pnmshear does the shearing by looping over the source pixels and distributing fractions to each of the destina-
       tion  pixels.   This has an 'anti-aliasing' effect - it avoids jagged edges and similar artifacts.  However, it
       also means that the original colors in the image are modified and there are typically more  of  them  than  you
       started  with.   If  you  need  to  keep precisely the same set of colors, see the -noantialias option.  If the
       expanded palette is a problem, you can run the result through pnmquant.

              This determines the color of the background on which the sheared image sits.

              Specify the color (color) as described for the argument of the ppm_parsecolor() library routine .

              By default, if you don't specify this option, pnmshear selects what appears to it to be  the  background
              color  of the original image.  It determines this color rather simplisticly, by taking an average of the
              colors of the two top corners of the image.

              This option was new in Netpbm 10.37 (December 2006).  Before that, pnmshear always  behaved  as  is  the
              default now.

              This  option  forces  pnmrotate  to simply move pixels around instead of synthesizing output pixels from
              multiple input pixels.  The latter could cause the output to contain colors that are not in  the  input,
              which may not be desirable.  It also probably makes the output contain a large number of colors.  If you
              need a small number of colors, but it doesn't matter if they are the exact ones from the input, consider
              using pnmquant on the output instead of using -noantialias.

              Note that to ensure the output does not contain colors that are not in the input, you also must consider
              the background color.  See the -background option.

       pnmrotate(1), pamflip(1), pnmquant(1), pnm(1)

       Copyright (C) 1989, 1991 by Jef Poskanzer.

netpbm documentation           27 November 2006        Pnmshear User Manual(0)