1Pambackground User Manual(0) Pambackground User Manual(0)
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6 pambackground - create a mask of the background area of an image
7
10 pambackground
11 [netpbmfile]
13 [-verbose=]
15 Minimum unique abbreviations of options are acceptable. You may use
17 double hyphens instead of single hyphen to denote options. You may use
18 white space in place of the equals sign to separate an option name from
19 its value.
20
24 This program is part of Netpbm(1).
25 pambackground reads a PNM or PAM image as input. It generates as out‐
27 put a PAM image that identifies the background area of the image (a
28 mask).
29 To identify the background, pambackground assumes the image is a fore‐
31 ground image, smaller than the total image size, placed over a single-
32 color background. It assumes that foreground image is solid -- it does
33 not have holes through which the background can be seen. So in spe‐
34 cific, pambackground first identifies the background color, then finds
35 all contiguous pixels of that color in regions touching any edge of the
36 image. Think of it as starting at each of the four edges and moving
37 inward and spreading out as far as possible until it hits pixels of
38 another color (the foreground image).
39 pambackground identifies the background color as follows: If any 3 cor‐
41 ners of the image are the same color, that's the background color. If
42 not, but 2 corners are the same color, the background color is the
43 color of a pair of identically colored corners in this priority order:
44 top, right, left, bottom. If no two corners have the same color, the
45 background color is the color of the upper left corner.
46 In a typical photograph, the area that you would consider the back‐
48 ground is many shades of a color, so to pambackground it is multiple
49 colors and pambackground will not meaningfully identify the background
50 of your image. To use pambackground in this case, you might use ppm‐
51 change to change all similar colors to a single one first. For exam‐
52 ple, if the photograph is a building against a blue sky, where nothing
53 remotely sky-blue appears in the building, you could use ppmchange to
54 change all pixels within 20% of "SkyBlue" to SkyBlue, then run pamback‐
55 ground on it.
56 You might even extract the argument for ppmchange from the image in
58 question, using ppmtoarbtxt. In the foregoing example, we knew the
59 background was approximately SkyBlue, but if we didn't we could just
60 get the color of the top left pixel, in a form suitable for the color
61 arguments of ppmchange like this:
62 $ echo "#(fred)/#(fgreen)/#(fblue)" >/tmp/bodyskl
64 $ color=$(pamcut 0 0 1 1 myimage.ppm | ppmtoarbtxt /tmp/bodyskl)
65 A more convenient means of dealing with a multi-shade background is to
67 use pnmquant to produce a version of the image with a very small number
68 of colors. The background would likely then be all one color.
69 If the pnmquant and ppmchange methods above do not adequately distin‐
71 guish foreground colors from background colors, you can try a more
72 elaborate method using pnmremap. If you can manually create a palette
73 with one color to which all the background pixels are similar, and
74 other colors to which the foreground pixels are similar, you can use it
75 as input to pnmremap to create a smarter version of what you get with
76 the pnmquant or ppmchange methods, so that pambackground is more likely
77 to separate background from foreground as your eye does.
78 The PAM that pambackground creates has a single plane, with a maxval of
80 1. The sample value 1 means background; 0 means foreground. There is
81 no tuple type. Some older programs (but none that are part of Netpbm)
82 don't know what a PAM is and expect a mask to be in the form of a PGM
83 or PBM image. To convert pambackground's output to PBM, use pamtopnm
84 -assume. To convert to PGM, use pgmtopgm.
85 netpbmfile is the file specification of the input file, or - to indi‐
87 cate Standard Input. The default is Standard Input.
88 A common use for a background mask is with pamcomp. You could replace
90 the entire background (or foreground) of your image with something
91 else.
92 Another common use is to make an image with the background transparent
94 (in some image format that has a concept of transparency) so that image
95 can be overlaid onto another image later. Netpbm's converters to image
96 formats that have transparency (e.g. PNG) let you use the mask that
97 pambackground generates to identify the transparent areas for the out‐
98 put. You can create a PAM image with transparency with pamstack.
99 To simply make a mask of all the areas of a specified color, use ppm‐
101 colormask. If you have a unique background color (one that doesn't
102 occur in the foreground) and know what it is, this can create a back‐
103 ground mask in cases that pambackground cannot: where there are see-
104 through holes in the foreground image.
105
109 -verbose
110 Tell interesting facts about the process.
111
116 $ pambackground test.ppm | pnminvert >/tmp/bgmask.pgm
117 $ pamcomp -alpha=bgmask.pgm test.ppm wallpaper.ppm >output.ppm
118 $ pnmquant 5 test.pgm | pambackground test.ppm >/tmp/bgmask.pam
119
123 ppmcolormask(1), pamcomp(1), ppmchange(1), pnmquant(1), pnmremap(1),
124 pamtopnm(1), pgmtopgm(1), pamstack(1), pnm(1), pam(1),
125
128 pambackground was new in Netpbm 10.37 (December 2006).
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131 This manual page was generated by the Netpbm tool 'makeman' from HTML
132 source. The master documentation is at
133 http://netpbm.sourceforge.net/doc/pambackground.html
135netpbm documentation 24 November 2014 Pambackground User Manual(0)