1Pambackground User Manual(0) Pambackground User Manual(0)
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6 pambackground - create a mask of the background area of an image
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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 as far as possible until it hits pixels of another color (the
38 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 In Release 10.37, pambackground does not really do what is promised
58 above. It can't see places where the background appears in the middle
59 of a row (think of the sky between two buildings). From Release 10.38
60 forward, it snakes through whatever passages it has to to find all the
61 background.
62 The PAM that pambackground creates has a single plane, with a maxval of
64 1. The sample value 1 means background; 0 means foreground. There is
65 no tuple type. Some older programs (but none that are part of Netpbm)
66 don't know what a PAM is and expect a mask to be in the form of a PGM
67 or PBM image. To convert pambackground's output to PBM, use pamtopnm
68 -assume. To convert to PGM, use pgmtopgm.
69 netpbmfile is the file specification of the input file, or - to indi‐
71 cate Standard Input. The default is Standard Input.
72 A common use for a background mask is with pamcomp. You could replace
74 the entire background (or foreground) of your image with something
75 else.
76 Another common use is to make an image with the background transparent
78 (in some image format that has a concept of transparency; not Netpbm
79 formats) so that image can be overlaid onto another image later.
80 Netpbm's converters to image formats that have transparency (e.g. PNG)
81 let you use the mask that pambackground generates to identify the
82 transparent areas for the output.
83 To simply make a mask of all the areas of a specified color, use ppm‐
85 colormask. If you have a unique background color (one that doesn't
86 occur in the foreground) and know what it is, this can create a back‐
87 ground mask in cases that pambackground cannot: where there are see-
88 through holes in the foreground image.
89
92 -verbose
93 Tell interesting facts about the process.
94
99 ppmcolormask(1), pamcomp(1), pamtopnm(1), pgmtopgm(1), pnm(1), pam(1),
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103 pambackground was new in Netpbm 10.37 (December 2006).
104netpbm documentation 31 December 2006 Pambackground User Manual(0)