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 ), 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 .

              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

netpbm documentation                              22 March 2020                          Pnmshear User Manual(1)