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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 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 fore-
ground 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 con-
tiguous 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 photo-
graph 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 ppm-
toarbtxt. 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:
$ echo "#(fred)/#(fgreen)/#(fblue)" >/tmp/bodyskl
$ color=$(pamcut 0 0 1 1 myimage.ppm | ppmtoarbtxt /tmp/bodyskl)
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 col-
ors 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 simi-
lar, 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 pro-
grams (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 back-
ground (or foreground) of your image with something else.
Another common use is to make an image with the background transparent (in some image for-
mat 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
-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), 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(0)
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