Provided by: netpbm_11.01.00-2build1_amd64 bug

NAME

       pamthreshold - threshold grayscale image to black and white

SYNOPSIS

       pamthreshold  [-simple]  [-local=widthxheight] [-dual=widthxheight] [-threshold=threshold]
       [-contrast=threshold] [inputpamfile]

       Minimum unique abbreviations of options are acceptable. You may use double hyphens instead
       of  a single hyphen to denote options. You may use white space in place of the equals sign
       to separate an option name from its value.

DESCRIPTION

       This program is part of Netpbm(1).

       pamthreshold thresholds a grayscale image. Thresholding  means  dividing  the  image  into
       background and foreground by comparing every pixel to a thresholding value.

       The input should be a PGM image or a PAM image of tuple type GRAYSCALE or GRAYSCALE_ALPHA.
       However, pamthreshold doesn't check; it just thresholds the first channel as  if  it  were
       grayscale  samples  and  if  there  is  a  second  channel,  processes  it  as  if it is a
       transparency (alpha) channel.  So if you feed it e.g.  a  PPM  image,  it  will  work  but
       produce probably useless results.

       The  output  is  a  PAM with tuple type BLACKANDWHITE or BLACKANDWHITE_ALPHA, depending on
       whether the input has a transparency channel.  You can turn this into a PBM (if  you  need
       to  use  it  with an older program that doesn't understand PAM, or you can't afford the 8X
       amount of space that PAM uses for the image) with pamtopnm.

       The output is to Standard Output.

       When the input has a transparency channel, pamthreshold includes a transparency channel in
       the  output.   Since  the  output has maxval 1, the transparency channel can indicate only
       fully transparent or fully opaque.  pamthreshold make it fully transparent where the input
       is more than half transparent and fully opaque where it isn't.

       The  transparency function was new in Netpbm 10.43 (June 2008).  Before that, pamthreshold
       ignores any transparency channel in the input.

       Another way to convert a grayscale image to black and white is to  dither.   Dithering  is
       using clustered black and white pixels such that if you step back and look at the picture,
       you see varying levels of gray.  pamditherbw does dithering.

OPTIONS

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

       Without any options, pamthreshold uses a method based on the iterative algorithm found  in
       the wikipedia ⟨http://www.wikipedia.org/⟩  article
        Thresholding                              (image                              processing)http://en.wikipedia.org/wiki/Thresholding_%28image_processing%29⟩      to   compute   the
       thresholding               value.                (               this              version
       ⟨http://en.wikipedia.org/w/index.php?title=Thresholding_%28image_processing%29&oldid=132306976⟩
         of  the  Wikipedia  article  was  current  at  the  time of this writing).  It uses this
       threshold to globally threshold the image.  This should work well for  most  images.   The
       program  issues  a  message  telling  you  what threshold it used.  (Netpbm messages go to
       Standard Error, and you can turn them off with the Netpbm common option -quiet).

       Options -simple, -local, and -dual select other methods.

       -simple
              This selects simple or global thresholding, i.e. pamthreshold compares every  pixel
              to the threshold you specify with -threshold.  Those with a brightness greater than
              or equal to the threshold become white; others become black.  This works  well  for
              black  and  white  text pages scanned with a flatbed scanner and is faster than the
              default method that iteratively determines the thresholding value first.

       -local=widthxheight
              This selects local adaptive thresholding (also known as dynamic thresholding) using
              the  neighborhood  which  is  the  square  width pixels wide and height pixels high
              centered  on  the  pixel  in  question.   pamthreshold   computes   the   threshold
              individually  for  each pixel of the image.  This can accommodate changing lighting
              conditions in the image.  Depending on the size of the  neighborhood  this  can  be
              quite slow.

       -dual=widthxheight
              This  selects  a  dual  thresholding  algorithm  using  a  global threshold for low
              contrast neighborhoods and local thresholding otherwise.  This can preserve  larger
              back-  respectively  foreground  areas  than  local  adaptive  thresholding.   This
              algorithm was proposed in the paper "An Approach To Licence Plate  Recognition"  by
              J.R. Parker and Pavol Federl.

       -threshold=threshold
              This  sets the thresholding value for simple or local thresholding.  The value is a
              floating point number in the range [0,  1]  directly  proportional  to  the  Netpbm
              sample values, where 0 corresponds to black and 1 to the maxval of the image.

              If you don't specify this option, pamthreshold uses a threshold of 0.5.

              Without -simple, -local, or -dual, this option is meaningless.

              The  meaning  of  the  threshold  depends  upon the kind of thresholding you do (as
              determined by other options).  Roughly, pixels at least as bright as the  threshold
              become white in the output while others become black.

       -contrast=threshold
              This  sets the threshold to determine if a neighborhood has low contrast or not for
              dual thresholding.  The value is a floating point number in the range [0, 1].

              If you don't specify this option, pamthreshold uses a contrast threshold  of  0.05.
              Without -dual this option is meaningless.

SEE ALSO

       pamditherbw(1), ppmtopgm(1), pamtopnm(1), pam(1)

HISTORY

       pamthreshold was new in Netpbm 10.34 (June 2006).

AUTHOR

       pamthreshold is Copyright © 2006 by Erik Auerswald and released under the GPL(1).

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/pamthreshold.html