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