1Pamthreshold User Manual(0) Pamthreshold User Manual(0)
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6 pamthreshold - threshold grayscale image to black and white
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10 pamthreshold [-simple] [-local=widthxheight] [-dual=widthxheight]
11 [-threshold=threshold] [-contrast=threshold] [inputpamfile]
12 Minimum unique abbreviations of options are acceptable. You may use
14 double hyphens instead of a single hyphen to denote options. You may
15 use white space in place of the equals sign to separate an option name
16 from its value.
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20 This program is part of Netpbm(1).
21 pamthreshold thresholds a grayscale image. Thresholding means dividing
23 the image into background and foreground by comparing every pixel to a
24 thresholding value.
25 The input should be a PGM image or a PAM image of tuple type GRAYSCALE
27 or GRAYSCALE_ALPHA. However, pamthreshold doesn't check; it just
28 thresholds the first channel as if it were grayscale samples and if
29 there is a second channel, processes it as if it is an alpha (trans‐
30 parency) channel. So if you feed it e.g. a PPM image, it will work but
31 produce probably useless results.
32 The output is a PAM with tuple type BLACKANDWHITE or BLACKAND‐
34 WHITE_ALPHA, depending on whether the input has an alpha channel. You
35 can turn this into a PBM (if you need to use it with an older program
36 that doesn't understand PAM, or you can't afford the 8X amount of space
37 that PAM uses for the image) with pamtopnm.
38 The output is to Standard Output.
40 When the input has an alpha channel, pamthreshold includes an alpha
42 channel in the output. Since the output has maxval 1, the alpha chan‐
43 nel can indicate only fully transparent or fully opaque. pamthreshold
44 make it fully transparent where the input is more than half transparent
45 and fully opaque where it isn't.
46 The alpha function was new in Netpbm 10.43 (June 2008). Before that,
48 pamthreshold ignores any alpha channel in the input.
49 Another way to convert a grayscale image to black and white is to
51 dither. Dithering is using clustered black and white pixels such that
52 if you step back and look at the picture, you see varying levels of
53 gray. pamditherbw does dithering.
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57 Without any options, pamthreshold uses an iterative algorithm found in
58 the wikipedia ⟨http://www.wikipedia.org/⟩ article
59 Thresholding (image processing)
60 ⟨http://en.wikipedia.org/wiki/Thresholding_%28image_processing%29⟩ to
61 compute the thresholding value. ( this version
62 ⟨http://en.wikipedia.org/w/index.php?title=Thresholding_%28image_processing%29&oldid=132306976⟩
63 of the Wikipedia article was current at the time of this writing). It
64 uses this threshold to globally threshold the image. This should work
65 well for most images. The program issues a message telling you what
66 threshold it used. (Netpbm messages go to Standard Error, and you can
67 turn them off with the Netpbm common option -quiet).
68 -simple
73 This selects simple or global thresholding, i.e. pamthreshold
74 compares every pixel to the threshold you specify with -thresh‐
75 old. Those with a brightness greater than or equal to the
76 threshold become white; others become black. This works well
77 for black and white text pages scanned with a flatbed scanner
78 and is faster than the default method that iteratively deter‐
79 mines the thresholding value first.
80 -local=widthxheight
83 This selects local adaptive thresholding (also known as dynamic
84 thresholding) using the neighborhood of width and height around
85 every pixel. pamthreshold computes the threshold individually
86 for each pixel of the image. This can accommodate changing
87 lighting conditions in the image. Depending on the size of the
88 neighborhood this can be quite slow.
89 -dual=widthxheight
92 This selects a dual thresholding algorithm using a global
93 threshold for low contrast neighborhoods and local thresholding
94 otherwise. This can preserve larger back- respectively fore‐
95 ground areas than local adaptive thresholding. This algorithm
96 was proposed in the paper "An Approach To Licence Plate Recogni‐
97 tion" by J.R. Parker and Pavol Federl.
98 -threshold=threshold
101 This sets the thresholding value for simple or local threshold‐
102 ing. The value is a floating point number in the range [0, 1]
103 directly proportional to the Netpbm sample values, where 0 cor‐
104 responds to black and 1 to the maxval of the image.
105 If you don't specify this option, pamthreshold uses a threshold
107 of 0.5. Without -simple or -local this option is meaningless.
108 The meaning of the threshold depends upon the kind of threshold‐
110 ing you do (as determined by other options). Roughly, pixels at
111 least as bright as the threshold become white in the output
112 while others become black.
113 -contrast=threshold
116 This sets the threshold to determine if a neighborhood has low
117 contrast or not for dual thresholding. The value is a floating
118 point number in the range [0, 1].
119 If you don't specify this option, pamthreshold uses a contrast
121 threshold of 0.05. Without -dual this option is meaningless.
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128 pamditherbw(1), ppmtopgm(1), pamtopnm(1), pam(1)
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132 pamthreshold was new in Netpbm 10.34 (June 2006).
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136 pamthreshold is Copyright © 2006 by Erik Auerswald and released under
137 the GPL(1).
138netpbm documentation 06 June 2007 Pamthreshold User Manual(0)