man > pambackground(1)

Pambackground User Manual(1)           General Commands Manual          Pambackground User Manual(1)

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 pix‐
       els 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  or‐
       der:  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 ppm‐
       change 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 pam‐
       background on it.

       You might even extract the argument for ppmchange from the image in question,  using  pamget‐
       color.  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  pro‐
       duce  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 manu‐
       ally  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 con‐
       vert 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  cre‐
       ate 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 ]8;;index.html#commonoptions\ Common Options]8;;\ ), 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),   pgm‐
       topgm(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)