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Pambackground User Manual(0)                      Pambackground User Manual(0)



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 back-
       ground 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 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  pam-
       background 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  pamback-
       ground on it.

       In Release 10.37, pambackground does not really do what is promised above.  It can't see places where the back-
       ground appears in the middle of a row (think of the sky between two buildings).  From Release 10.38 forward, it
       snakes through whatever passages it has to to find all the background.

       The  PAM  that  pambackground  creates  has a single plane, with a maxval of 1.  The sample value 1 means back-
       ground; 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  Stan-
       dard 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; not Netpbm formats) so that image can be overlaid onto another image later.  Netpbm's convert-
       ers 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.

       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
       -verbose
              Tell interesting facts about the process.




SEE ALSO
       ppmcolormask(1), pamcomp(1), pamtopnm(1), pgmtopgm(1), pnm(1), pam(1),


HISTORY
       pambackground was new in Netpbm 10.37 (December 2006).



netpbm documentation           31 December 2006   Pambackground User Manual(0)