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1 perry 1 pnmhisteq
2 !!!pnmhisteq
3 NAME
4 SYNOPSIS
5 DESCRIPTION
6 OPTIONS
7 BUGS
8 SEE ALSO
9 AUTHOR
10 ----
11 !!NAME
12
13
14 pnmhisteq - histogram equalise a portable anymap
15 !!SYNOPSIS
16
17
18 __pnmhisteq__ [[__-gray__] [[__-rmap__
19 ''pgmfile''] [[__-wmap__ ''pgmfile'']
20 [[__-verbose__] [[''pnmfile'']
21 !!DESCRIPTION
22
23
24 __pnmhisteq__ increases the contrast of a portable
25 graymap or pixmap through the technique of ''histogram
26 equalisation''[[1]. A histogram of the luminance of pixels
27 in the map is computed, from which a transfer function is
28 calculated which spreads out intensity levels around
29 histogram peaks and compresses them at troughs. This has the
30 effect of using the available levels of intensity more
31 efficiently and thereby increases the detail visible in the
32 image.
33
34
35 Mathematically, if ''N[[i]'' is the number of pixels of
36 luminosity ''i'' in the image and ''T'' is the total
37 number of pixels, luminosity ''j'' is replaced
38 by:
39
40
41 j --- \
42
43
44 If you're processing a related set of images, for example
45 frames of an animation, it's generally best to apply the
46 same intensity map to every frame, since otherwise you'll
47 get distracting frame-to-frame changes in the brightness of
48 objects. __pnmhisteq__'s __-wmap__ option allows you
49 to save, as a portable graymap, the luminosity map computed
50 from an image (usually a composite of the images you intend
51 to process created with __pnmcat__). Then, you can
52 subsequently process each of the individual images using the
53 luminosity map saved in the file, supplied with the
54 __-rmap__ option.
55 !!OPTIONS
56
57
58 __-gray__
59
60
61 When processing a pixmap, only gray pixels (those with
62 identical red, green, and blue values) are included in the
63 histogram and modified in the output image. This is a
64 special purpose option intended for images where the actual
65 data are gray scale, with colour annotations you don't want
66 modified. Weather satellite images that show continent
67 outlines in colour are best processed using this option. The
68 option has no effect when the input is a
69 graymap.
70
71
72 __-rmap__ ''mapfile''
73
74
75 Process the image using the luminosity map specified by the
76 portable graymap ''mapfile.'' The graymap, usually
77 created by an earlier run of __pnmhisteq__ with the
78 __-wmap__ option, contains a single row with number of
79 columns equal to the ''maxval'' (greatest intensity) of
80 the image. Each pixel in the image is transformed by looking
81 up its luminosity in the corresponding column in the map
82 file and changing it to the value given by that
83 column.
84
85
86 __-wmap__ ''mapfile''
87
88
89 Creates a portable graymap, ''mapfile,'' containing the
90 luminosity map computed from the histogram of the input
91 image. This map file can be read on subsequent runs of
92 __pnmhisteq__ with the __-rmap__ option, allowing a
93 group of images to be processed with an identical
94 map.
95
96
97 __-verbose__
98
99
100 Prints the histogram and luminosity map on standard
101 error.
102
103
104 All flags can be abbreviated to their shortest unique
105 prefix.
106 !!BUGS
107
108
109 Histogram equalisation is effective for increasing the
110 visible detail in scientific imagery and in some
111 continuous-tone pictures. It is often too drastic, however,
112 for scanned halftone images, where it does an excellent job
113 of making halftone artifacts apparent. You might want to
114 experiment with __pgnnorm__, __ppmnorm__, and
115 __pnmgamma__ for more subtle contrast
116 enhancement.
117
118
119 The luminosity map file supplied by the __-rmap__ option
120 must have the same ''maxval'' as the input image. This is
121 always the case when the map file was created by the
122 __-wmap__ option of __pnmhisteq__. If this restriction
123 causes a problem, simply adjust the ''maxval'' of the map
124 with __pnmdepth__ to agree with the input
125 image.
126
127
128 If the input is a PBM file (on which histogram equalisation
129 is an identity operation), the only effect of passing the
130 file through __pnmhisteq__ will be the passage of
131 time.
132 !!SEE ALSO
133
134
135 pgmnorm(1), pnm(5), pnmcat(1),
136 pnmdepth(1), pnmgamma(1),
137 pnmnorm(1)
138
139
140 [[1]
141
142
143 Russ, John C. The Image Processing Handbook. Boca Raton: CRC
144 Press, 1992. Pages 105-110.
145 !!AUTHOR
146 Copyright (C) 1995 by John Walker
147 (kelvin@fourmilab.ch).
148 WWW home page: http://www.fourmilab.ch/
149 Permission to use, copy, modify, and distribute this
150 software and its documentation for any purpose and without
151 fee is hereby granted, without any conditions or
152 restrictions. This software is provided ``as is'' without
153 express or implied warranty.
154 ----
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