Pambackground User Manual(1)General Commands ManuaPambackground 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 out-
put a PAM image that identifies the background area of the image (a
mask).
To identify the background, pambackground assumes the image is a fore-
ground 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 spe-
cific, 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 an-
other color (the foreground image).
pambackground identifies the background color as follows: If any 3 cor-
ners 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 back-
ground 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 exam-
ple, 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.
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 distin-
guish 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 indi-
cate 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 out-
put. You can create a PAM image with transparency with pamstack.
To simply make a mask of all the areas of a specified color, use ppm-
colormask. If you have a unique background color (one that doesn't oc-
cur in the foreground) and know what it is, this can create a back-
ground 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 ⟨index.html#commonoptions⟩ ), 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)
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