Manual: pnmshear

Pnmshear User Manual(1)				   General Commands Manual			     Pnmshear User Manual(1)

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 prob‐
       lem, 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 ), pn‐
       mshear 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)