1Pnmhisteq User Manual(0) Pnmhisteq User Manual(0)
2
6 pnmhisteq - histogram equalize a PNM image
7
10 pnmhisteq
11 [-gray]
13 [-noblack] [-nowhite]
15 [-rmap pgmfile]
17 [-wmap pgmfile]
19 [-verbose]
21 [pnmfile]
23
27 This program is part of Netpbm(1).
28 pnmhisteq increases the contrast of a PGM or PPM image through the
30 technique of "histogram equalization."[1]
31 pnmhisteq computes a histogram of the luminosity of the pixels in the
33 image. It then calculates a mapping between each luminosity and a new
34 luminosity such that it spreads out intensity levels around histogram
35 peaks and compresses them at troughs. I.e. it moves pixels around in
36 the histogram so as to make it flat. It applies that mapping to the
37 input image to produce the output image. The effect of this is that
38 the image has equal numbers of pixels at each possible intensity level,
39 which means it uses the available levels of intensity more efficiently
40 and thereby has more visible detail.
41 Mathematically, the luminosity mapping is this: Assume the pixels are
43 sorted by luminosity into B buckets numbered from 0 (lowest luminosity)
44 to B-1. N[i] is the number of pixels in bucket i. T is the total num‐
45 ber of pixels (sum of N[i] over all i). W is the luminosity of white.
46 pnmhisteq replaces an input pixel whose luminosity falls into bucket j
48 with one whose luminosity is:
49 j
52 ---
53 \
54 > (N[i] / T) * W
55 /
56 ---
57 i=0
58 Considering a grayscale image for simplicity, this means that pixels in
61 the most luminous bucket become white. Pixels in the 10th per centile
62 of luminosity become 10% of white.
63 pnmhisteq maps a single luminosity in the input to a single luminosity
65 in the output. That means if pixels A and B both have luminosity .2 in
66 the input, and pixel A has luminosity .4 in the output, pixel B also
67 has luminosity .4 in the output. And since the luminosities in the in‐
68 put are not continuous, the luminosities in the output aren't either
69 and pnmhisteq doesn't meet the ideal of having exactly the same number
70 of pixels of each luminosity in the output.
71 If you're processing a related set of images, for example frames of an
73 animation, it's generally best to apply the same luminosity mapping to
74 every frame, since otherwise you'll get distracting frame-to-frame
75 changes in the brightness of objects. pnmhisteq's -wmap option allows
76 you to save, as a PGM image, the luminosity map it computes from an im‐
77 age. The -rmap option causes pnmisteq to use such an image as its lu‐
78 minosity map.
79 So you can run pnmhisteq with -wmap on a composite you created with pn‐
81 mcat of the images you intend to process. Then, you can run pnmisteq
82 with -rmap on each of the individual images, using the luminosity map
83 you generated from the composite.
84 Use pnmhistmap to see the result. Run a color image through ppmtopgm
86 first so that you see a histogram of the luminosity instead of his‐
87 tograms of the three color components. It should generally show a flat
88 histogram. But because of the quantization effects described above,
89 you might see high bars interleaved with low bars, with the local aver‐
90 age being flat. To see local averages, use the -width option of pn‐
91 mhistmap.
92
96 In addition to the options common to all programs based on libnetpbm
97 (most notably -quiet, see
98 Common Options ⟨index.html#commonoptions⟩ ), pnmhisteq recognizes the
99 following command line options:
100 You can abbreviate any option to its shortest unique prefix.
102 -gray When processing a color image, only gray pixels (those with
106 identical red, green, and blue values) are included in the his‐
107 togram and modified in the output image. This is a special pur‐
108 pose option intended for images where the actual data are gray
109 scale, with color annotations you don't want modified. Weather
110 satellite images that show continent outlines in color are best
111 processed using this option. The option has no effect when the
112 input is a graymap.
113 -noblack
116 Do not include black pixels in the equalization. The black pix‐
117 els in the output are exactly the black pixels in the input and
118 the number of black pixels does not affect the color of any
119 other pixels.
120 Sometimes, black isn't as much a color as a background or anno‐
122 tation for the real colors, so you want to treat it specially
123 this way. Think of a picture of stars, which is nearly all
124 black, but with lots of stars of different brightness. You want
125 to change the brightnesses of the stars to maximize contrast be‐
126 tween them, but if you considered the blackness to be signifi‐
127 cant, all the stars would end up close to full white.
128 This option was new in Netpbm 10.70 (March 2015).
130 -nowhite
133 Same as -noblack, but for the white pixels.
135 This option was new in Netpbm 10.70 (March 2015).
137 -rmap mapfile
140 Process the image using the luminosity map specified by the PGM
141 file mapfile.
142 The PGM image, usually created by an earlier run of pnmhisteq
144 with the -wmap option, contains a single row with number of col‐
145 umns equal to the maxval (greatest intensity value) of the image
146 plus one. Each pixel in the image is transformed by looking up
147 its luminosity in the corresponding column in the map file (col‐
148 umn number = luminosity) and changing it to the value given by
149 that column.
150 -wmap mapfile
153 Creates a PGM file mapfile, containing the luminosity map com‐
154 puted from the histogram of the input image. This map file can
155 be read on subsequent runs of pnmhisteq with the -rmap option,
156 allowing a group of images to be processed with an identical
157 map.
158 -verbose
161 Prints the histogram and luminosity map on Standard Error.
162
167 Histogram equalization is effective for increasing the visible detail
168 in scientific imagery and in some continuous-tone pictures. It is of‐
169 ten too drastic, however, for scanned halftone images, where it does an
170 excellent job of making halftone artifacts apparent. You might want to
171 experiment with pnmnorm and pnmgamma for more subtle contrast enhance‐
172 ment.
173 The luminosity map file supplied by the -rmap option must have the same
175 maxval as the input image. This is always the case when the map file
176 was created by the -wmap option of pnmhisteq. If this restriction
177 causes a problem, simply adjust the maxval of the map with pamdepth to
178 agree with the input image.
179 If the input is a PBM file (on which histogram equalization is an iden‐
181 tity operation), the only effect of passing the file through pnmhisteq
182 will be the passage of time.
183
186 pnmnorm(1), pnmcat(1), pamdepth(1), pnmgamma(1), pnm(1),
187 [1] Russ, John C. The Image Processing Handbook. Boca Raton: CRC
191 Press, 1992. Pages 105-110.
192
194 This manual page was generated by the Netpbm tool 'makeman' from HTML
195 source. The master documentation is at
196 http://netpbm.sourceforge.net/doc/pnmhisteq.html
198netpbm documentation 22 March 2015 Pnmhisteq User Manual(0)