NAME

       pambackground - create a mask of the background area of an image

SYNOPSIS

       pambackground

       [netpbmfile]

       [-verbose]

       Minimum  unique  abbreviations  of  options are acceptable.  You may use double hyphens instead of 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).

       pambackground reads a PNM or PAM image as input.  It generates as output a PAM image that identifies  the
       background area of the image (a mask).

       To identify the background, pambackground assumes the image is a foreground image, smaller than the total
       image  size, placed over a single-color background.  It assumes that foreground image is solid -- it does
       not have holes through which the background can be seen.  So in specific, pambackground first  identifies
       the  background color, then finds all contiguous pixels of that color in regions touching any edge of the
       image.  Think of it as starting at each of the four edges and moving inward and spreading out as  far  as
       possible until it hits pixels of another color (the foreground image).

       pambackground  identifies  the  background  color  as follows: If any 3 corners of the image are the same
       color, that's the background color.  If not, but 2 corners are the same color, the  background  color  is
       the  color of a pair of identically colored corners in this priority order: top, right, left, bottom.  If
       no two corners have the same color, the background color is the color of the upper left corner.

       In a typical photograph, the area that you would consider the background is many shades of a color, so to
       pambackground it is multiple colors and pambackground will not meaningfully identify  the  background  of
       your image.  To use pambackground in this case, you might use ppmchange to change all similar colors to a
       single  one  first.   For  example,  if  the  photograph  is a building against a blue sky, where nothing
       remotely sky-blue appears in the building, you could use ppmchange to change all  pixels  within  20%  of
       "SkyBlue" to SkyBlue, then run pambackground on it.

       You  might even extract the argument for ppmchange from the image in question, using pamgetcolor.  In the
       foregoing example, we knew the background was approximately SkyBlue, but if we didn't we could  just  get
       the color of the top left pixel, in a form suitable for the color arguments of ppmchange like this:

           $ color=$(pamgetcolor 0,0 -infile=/tmp/bodyskl|cut --fields=2 -delim=' ')

       A  more convenient means of dealing with a multi-shade background is to use pnmquant to produce a version
       of the image with a very small number of colors.  The background would likely then be all one color.

       If the pnmquant and ppmchange  methods  above  do  not  adequately  distinguish  foreground  colors  from
       background  colors,  you  can  try  a more elaborate method using pnmremap.  If you can manually create a
       palette with one color to which all the background pixels are similar, and  other  colors  to  which  the
       foreground  pixels  are  similar, you can use it as input to pnmremap to create a smarter version of what
       you get with the pnmquant or ppmchange  methods,  so  that  pambackground  is  more  likely  to  separate
       background from foreground as your eye does.

       The  PAM  that  pambackground  creates  has a single plane, with a maxval of 1.  The sample value 1 means
       background; 0 means foreground.  There is no tuple type.  Some older programs (but none that are part  of
       Netpbm)  don't  know what a PAM is and expect a mask to be in the form of a PGM or PBM image.  To convert
       pambackground's output to PBM, use pamtopnm -assume.  To convert to PGM, use pgmtopgm.

       netpbmfile is the file specification of the input file, or - to indicate Standard Input.  The default  is
       Standard Input.

       A  common  use  for  a  background  mask  is  with  pamcomp.  You could replace the entire background (or
       foreground) of your image with something else.

       Another common use is to make an image with the background transparent (in some image format that  has  a
       concept  of transparency) so that image can be overlaid onto another image later.  Netpbm's converters to
       image formats that have transparency (e.g. PNG) let you use the  mask  that  pambackground  generates  to
       identify  the  transparent  areas  for  the  output.   You  can create a PAM image with transparency with
       pamstack.

       To simply make a mask of all the areas of a specified color, use ppmcolormask.   If  you  have  a  unique
       background  color  (one  that  doesn't  occur  in  the foreground) and know what it is, this can create a
       background mask in cases that pambackground cannot: where there are see-through holes in  the  foreground
       image.

OPTIONS

       In  addition  to  the options common to all programs based on libnetpbm (most notably -quiet, see  Common
       Options ), pambackground recognizes the following command line option:

       -verbose
              Tell interesting facts about the process.

EXAMPLES

           $ pambackground test.ppm | pnminvert >/tmp/bgmask.pgm
           $ pamcomp -alpha=bgmask.pgm test.ppm wallpaper.ppm >output.ppm

           $ pnmquant 5 test.pgm | pambackground test.ppm >/tmp/bgmask.pam

SEE ALSO

       ppmcolormask(1),  pamcomp(1),   ppmchange(1),   pnmquant(1),   pnmremap(1),   pamtopnm(1),   pgmtopgm(1),
       pamstack(1), pamgetcolor(1), pbmmaskd(1), pnm(1), pam(1),

HISTORY

       pambackground was new in Netpbm 10.37 (December 2006).

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/pambackground.html

netpbm documentation                            24 November 2014                    Pambackground User Manual(1)