1Pamditherbw User Manual(0) Pamditherbw User Manual(0)
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6 pamditherbw - dither grayscale image to black and white
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10 pamditherbw
11 [-floyd | -fs | -atkinson | -threshold | -hilbert | -dither8 | -d8 |
13 -cluster3 | -c3 | -cluster4 | -c4 | -cluster8 | -c8]
14 [-value val]
16 [-clump size]
18 [-randomseed=integer]
20 [pamfile]
22 All options can be abbreviated to their shortest unique prefix.
24
27 This program is part of Netpbm(1).
28 pamditherbw dithers a grayscale image. Dithering means turning each
30 shade of gray into a pattern of black and white pixels that, from a
31 distance, look the same as the gray.
32 The input should be a PGM image or a PAM image of tuple type GRAYSCALE.
34 However, pamditherbw doesn't check, so if you feed it e.g. a PPM image,
35 it will produce arbitrary results (actually, it just takes the first
36 channel of whatever you give it and treats it as if it represented gray
37 levels).
38 The output is a PAM with tuple type BLACKANDWHITE. You can turn this
40 into a PBM (if you need to use it with an older program that doesn't
41 understand PAM) with pamtopnm.
42 To do the opposite of dithering, you can usually just scale the image
44 down and then back up again with pamscale, possibly smoothing or blur‐
45 ring the result with pnmsmooth or pnmconvol. Or use the special case
46 program pbmtopgm.
47 To dither a color image (to reduce the number of pixel colors), use
49 ppmdither.
50 Another way to convert a grayscale image to a black and white image is
52 thresholding. Thresholding is simply replacing each grayscale pixel
53 with a black or white pixel depending on whether its brightness is
54 above or below a threshold. That threshold might vary. Simple thresh‐
55 olding is a degenerate case of dithering, so pamditherbw does very sim‐
56 ple thresholding with its -threshold option. But pamthreshold does
57 more sophisticated thresholding.
58 If all you want is to change a PGM image with maxval 1 to a PBM image,
60 pamtopnm will do that.
61
65 In addition to the options common to all programs based on libnetpbm
66 (most notably -quiet, see
67 Common Options ⟨index.html#commonoptions⟩ ), pamditherbw recognizes
68 the following command line options:
69 Quantization Method
72 The default quantization method is boustrophedonic Floyd-Steinberg
73 error diffusion (-floyd or -fs).
74 Also available are simple thresholding (-threshold); Bayer's ordered
76 dither (-dither8) with a 16x16 matrix;
77 Atkinson
78 ⟨http://www.tinrocket.com/projects/programming/graphics/00158/⟩ ; and
79 three different sizes of 45-degree clustered-dot dither (-cluster3,
80 -cluster4, -cluster8).
81 A space filling curve halftoning method using the Hilbert curve is also
83 available (-hilbert).
84 Floyd-Steinberg or Atkinson will almost always give the best looking
86 results; however, looking good is not always what you want. For
87 instance, you can use thresholding in a pipeline with the pnmconvol,
88 for tasks such as edge and peak detection. And clustered-dot dithering
89 gives a newspaper-ish look, a useful special effect.
90 Floyd-Steinberg is by far the more traditional, but
92 some claim
93 ⟨http://www.tinrocket.com/projects/programming/graphics/00158/⟩ Atkin‐
94 son works better.
95 The Hilbert curve method is useful for processing images before display
97 on devices that do not render individual pixels distinctly (like laser
98 printers). This dithering method can give better results than the
99 dithering usually done by the laser printers themselves. The -clump
100 option alters the number of pixels in a clump. Typically a PGM image
101 will have to be scaled to fit on a laser printer page (2400 x 3000 pix‐
102 els for an A4 300 dpi page), and then dithered to a PBM image before
103 being converted to a postscript file. A printing pipeline might look
104 something like:
105 pamscale -xysize 2400 3000 image.pgm | pamditherbw -hilbert | \
107 pamtopnm | pnmtops -scale 0.25 > image.ps
108 Other Options
111 -value This option alters the thresholding value for Floyd-Steinberg,
112 Atkinson, and simple thresholding. It should be a real number
113 between 0 and 1. Above 0.5 means darker images; below 0.5 means
114 lighter.
115 -clump This option alters the number of pixels in a clump when the
118 Hilbert curve method is used. This is usually an integer
119 between 2 and 100 (default 5). Smaller clump sizes smear the
120 image less and are less grainy, but seem to lose some grey scale
121 linearity.
122 -randomseed=integer
125 The Floyd-Steiberg and Atkinson methods use random numbers to
126 diffuse the error. This is the seed for the random number gen‐
127 erator. The other methods do not employ random numbers and
128 ignore this option.
129 Use this to ensure you get the same image on separate invoca‐
131 tions.
132 By default, pamditherbw uses a seed derived from the time of day
134 and process ID, which gives you fairly uncorrelated results in
135 multiple invocations.
136 This option was new in Netpbm 10.45 (December 2008).
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144 The only reference you need for this stuff is "Digital Halftoning" by
145 Robert Ulichney, MIT Press, ISBN 0-262-21009-6.
146 The Hilbert curve space filling method is taken from "Digital Halfton‐
148 ing with Space Filling Curves" by Luiz Velho, Computer Graphics Volume
149 25, Number 4, proceedings of SIGRAPH '91, page 81. ISBN 0-89791-436-8
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153 pamtopnm(1), pgmtopgm(1), pbmtopgm(1), pamthreshold(1), pbmreduce(1),
154 pnmconvol(1), pamscale(1), pam(1), pnm(1),
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158 pamditherbw was new in Netpbm 10.23 (July 2004), but is essentially the
159 same program as pgmtopbm that has existed practically since the begin‐
160 ning. pamditherbw differs from its predecessor in that it properly
161 adds brightnesses (using gamma transformations; pgmtopbm just adds them
162 linearly) and that it accepts PAM input in addition to PGM and PBM and
163 produces PAM output.
164 pamditherbw obsoletes pgmtopbm.
166 -atkinson was new in Netpbm 10.38 (March 2007).
168
171 Copyright (C) 1989 by Jef Poskanzer.
172
174 This manual page was generated by the Netpbm tool 'makeman' from HTML
175 source. The master documentation is at
176 http://netpbm.sourceforge.net/doc/pamditherbw.html
178netpbm documentation 10 May 2010 Pamditherbw User Manual(0)