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.
17
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 a transparency
30 (alpha) 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 a transparency channel.
35 You can turn this into a PBM (if you need to use it with an older pro‐
36 gram that doesn't understand PAM, or you can't afford the 8X amount of
37 space that PAM uses for the image) with pamtopnm.
38 The output is to Standard Output.
40 When the input has a transparency channel, pamthreshold includes a
42 transparency channel in the output. Since the output has maxval 1, the
43 transparency channel can indicate only fully transparent or fully
44 opaque. pamthreshold make it fully transparent where the input is more
45 than half transparent and fully opaque where it isn't.
46 The transparency function was new in Netpbm 10.43 (June 2008). Before
48 that, pamthreshold ignores any transparency 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 In addition to the options common to all programs based on libnetpbm
58 (most notably -quiet, see
59 Common Options ⟨index.html#commonoptions⟩ ), pamthreshold recognizes
60 the following command line options:
61 Without any options, pamthreshold uses a method based on the iterative
63 algorithm found in the wikipedia ⟨http://www.wikipedia.org/⟩ article
64 Thresholding (image processing)
65 ⟨http://en.wikipedia.org/wiki/Thresholding_%28image_processing%29⟩ to
66 compute the thresholding value. ( this version
67 ⟨http://en.wikipedia.org/w/index.php?title=Thresholding_%28image_processing%29&oldid=132306976⟩
68 of the Wikipedia article was current at the time of this writing). It
69 uses this threshold to globally threshold the image. This should work
70 well for most images. The program issues a message telling you what
71 threshold it used. (Netpbm messages go to Standard Error, and you can
72 turn them off with the Netpbm common option -quiet).
73 Options -simple, -local, and -dual select other methods.
75 -simple
80 This selects simple or global thresholding, i.e. pamthreshold
81 compares every pixel to the threshold you specify with -thresh‐
82 old. Those with a brightness greater than or equal to the
83 threshold become white; others become black. This works well
84 for black and white text pages scanned with a flatbed scanner
85 and is faster than the default method that iteratively deter‐
86 mines the thresholding value first.
87 -local=widthxheight
90 This selects local adaptive thresholding (also known as dynamic
91 thresholding) using the neighborhood which is the square width
92 pixels wide and height pixels high centered on the pixel in
93 question. pamthreshold computes the threshold individually for
94 each pixel of the image. This can accommodate changing lighting
95 conditions in the image. Depending on the size of the neighbor‐
96 hood this can be quite slow.
97 -dual=widthxheight
100 This selects a dual thresholding algorithm using a global
101 threshold for low contrast neighborhoods and local thresholding
102 otherwise. This can preserve larger back- respectively fore‐
103 ground areas than local adaptive thresholding. This algorithm
104 was proposed in the paper "An Approach To Licence Plate Recogni‐
105 tion" by J.R. Parker and Pavol Federl.
106 -threshold=threshold
109 This sets the thresholding value for simple or local threshold‐
110 ing. The value is a floating point number in the range [0, 1]
111 directly proportional to the Netpbm sample values, where 0 cor‐
112 responds to black and 1 to the maxval of the image.
113 If you don't specify this option, pamthreshold uses a threshold
115 of 0.5.
116 Without -simple, -local, or -dual, this option is meaningless.
118 The meaning of the threshold depends upon the kind of threshold‐
120 ing you do (as determined by other options). Roughly, pixels at
121 least as bright as the threshold become white in the output
122 while others become black.
123 -contrast=threshold
126 This sets the threshold to determine if a neighborhood has low
127 contrast or not for dual thresholding. The value is a floating
128 point number in the range [0, 1].
129 If you don't specify this option, pamthreshold uses a contrast
131 threshold of 0.05. Without -dual this option is meaningless.
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138 pamditherbw(1), ppmtopgm(1), pamtopnm(1), pam(1)
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142 pamthreshold was new in Netpbm 10.34 (June 2006).
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146 pamthreshold is Copyright © 2006 by Erik Auerswald and released under
147 the GPL(1).
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150 This manual page was generated by the Netpbm tool 'makeman' from HTML
151 source. The master documentation is at
152 http://netpbm.sourceforge.net/doc/pamthreshold.html
154netpbm documentation 06 June 2007 Pamthreshold User Manual(0)