NAME

       pnmshear - shear a PNM image by a specified angle

SYNOPSIS

       pnmshear

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

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

DESCRIPTION

       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.  In fact, if it gets too  close,  the  width  will  be  so  large  that
       pnmshear  cannot  do  computations in the word sizes it uses, and the program detects this
       and fails.

       pnmshear does the shearing 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 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.

OPTIONS

       In addition to the options common to all programs based on libnetpbm (most notably -quiet,
       see
        Common Options ⟨index.html#commonoptions⟩ ), pnmshear recognizes  the  following  command
       line options:

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

              Specify  the  color  (color)  as described for the argument of the pnm_parsecolor()
              library routine ⟨libnetpbm_image.html#colorname⟩ .

              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
              simplistically, 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.

       -noantialias
              This option forces pnmshear 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.

SEE ALSO

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

AUTHOR

       Copyright (C) 1989, 1991 by Jef Poskanzer.

DOCUMENT SOURCE

       This manual page was generated by the Netpbm tool 'makeman' from HTML source.  The  master
       documentation is at

              http://netpbm.sourceforge.net/doc/pnmshear.html