1Pnmnorm User Manual(0) Pnmnorm User Manual(0)
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6 pnmnorm - normalize the contrast in a Netbpm image
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10 pnmnorm
11 [-bpercent=N | -bvalue=N]
13 [-wpercent=N | -wvalue=N]
15 [-maxexpand]
17 [-keephues]
19 [-luminosity | -colorvalue | -saturation]
21 [ppmfile]
23 All options can be abbreviated to their shortest unique prefix. You
25 may use two hyphens instead of one to designate an option. You may use
26 either white space or an equals sign between an option name and its
27 value.
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31 This program is part of Netpbm(1).
32 pnmnorm reads a PNM image (PBM, PGM, or PPM). It normalizes the con‐
34 trast by forcing the brightest pixels to white, the darkest pixels to
35 black, and linearly rescaling the ones in between; and produces the
36 same kind of file as output. This is pretty useless for a PBM image.
37 The program first determines a mapping of old brightness to new bright‐
39 ness. For each possible brightness of a pixel, the program determines
40 a corresponding brightness for the output image.
41 Then for each pixel in the image, the program computes a color which
43 has the desired output brightness and puts that in the output. With a
44 color image, it is not always possible to compute such a color and
45 retain any semblance of the original hue, so the brightest and dimmest
46 pixels may only approximate the desired brightness.
47 For a PPM image, you have a choice of three different ways to define
49 brightness:
50 · luminosity
53 · color value
55 · saturation
57 In the case of saturation, 'brightness' is pretty much a mis‐
60 nomer, but you can use the brightness analogy to see what it
61 does. In the analogy, bright means saturated and dark means
62 unsaturated.
63 Note that all of these are different from separately normalizing the
65 individual color components.
66 An alternative way to spread out the brightnesses in an image is
68 pnmhisteq. pnmhisteq stretches the brightest pixels to white and the
69 darkest pixels to black, but rather than linearly adjusting the ones in
70 between, it adjusts them so that there are an equal number of pixels of
71 each brightness throughout the range. This gives you more contrast
72 than pnmnorm does, but can considerably change the picture in exchange.
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77 By default, the darkest 2 percent of all pixels are mapped to black,
78 and the brightest 1 percent are mapped to white. You can override
79 these percentages by using the -bpercent and -wpercent options, or you
80 can specify the exact pixel values to be mapped by using the -bvalue
81 and -wvalue options. You can get appropriate numbers for the options
82 from ppmhist. If you just want to enhance the contrast, then choose
83 values at elbows in the histogram; e.g. if value 29 represents 3% of
84 the image but value 30 represents 20%, choose 30 for bvalue. If you
85 want to brighten the image, then set bvalue to 0 and just fiddle with
86 wvalue; similarly, to darken the image, set wvalue to maxval and play
87 with bvalue.
88 If you specify both -bvalue and -bpercent, pnmnorm uses the one that
90 produces the least change. The same goes for -wvalue and -wpercent.
91 If you want to maximize the change instead of minimizing it, just cas‐
93 cade two runs of pnmnorm, specifying values for the first and percent‐
94 ages for the second.
95 This is further constrained by the -maxexpand option. Sometimes, too
97 much contrast is a bad thing. If your intensities are all concentrated
98 in the middle, -bpercent=2 and -wpercent=1 might mean that an intensity
99 of 60 gets stretched up to 100 and and intensity of 40 gets stretched
100 down to zero, for a range expansion of 150% (from a range of 40 to a
101 range of 100). That much stretching means two adjacent pixels that
102 used to differ in intensity by 4 units now differ by 10, and that might
103 be unsightly.
104 So that you can put a limit on the amount of expansion without having
106 to examine the image first, there is the -maxexpand option. It speci‐
107 fies the maximum expansion you will tolerate, as an additional per cen‐
108 tage. In the example above, you could say -maxexpand=50 to say you
109 want the range to expand by at most 50%, regardless of your other
110 options. pnmnorm figures out what intensity to stretch to full inten‐
111 sity and what intensity to stretch to zero intensity as described
112 above, and then raises the former and lowers the latter as needed to
113 limit the expansion to the amount you specified.
114 When pnmnorm limits the expansion due to -maxexpand, it tells you about
116 it with a message like this:
117 limiting expansion of 150% to 50%
118 In any case, pnmnorm tells you exactly what expansion it's doing, like
120 this:
121 remapping 25..75 to 0..100
123 Before Netpbm 10.26 (December 2004), it was not valid to specify both
125 -bvalue and -bpercent or -wvalue and -wpercent.
126 -maxexpand was new in Netpbm 10.32 (February 2006).
128 The -keephues option says to keep each pixel the same hue as it is in
130 the input; just adjust its brightness. You normally want this; the
131 only reason it is not the default behavior is backward compatibility
132 with a design mistake.
133 By default, pnmnorm normalizes contrast in each component independently
135 (except that the meaning of the -wpercent and -bpercent options are
136 based on the overall brightnesses of the colors, not each component
137 taken separately). So if you have a color which is intensely red but
138 dimly green, pnmnorm would make the red more intense and the green less
139 intense, so you end up with a different hue than you started with.
140 If you specify -keephues, pnmnorm would likely leave this pixel alone,
142 since its overall brightness is medium.
143 -keephues can cause clipping, because a certain color may be below a
145 target intensity while one of its components is saturated. Where
146 that's the case, pnmnorm uses the maximum representable intensity for
147 the saturated component and the pixel ends up with less overall inten‐
148 sity, and a different hue, than it is supposed to have.
149 This option is meaningless on grayscale images.
151 When you don't specify -keephues, the -luminosity, -colorvalue, and
153 -saturation options affect the transfer function (which is the same for
154 all three RGB components), but are meaningless when it comes to apply‐
155 ing the transfer function (since it is applied to each individual RGB
156 component).
157 Before Netpbm 9.25 (March 2002), there was no -keephues option.
159 -luminosity, -colorvalue, and -saturation determine what property of
161 the pixels pnmnorm normalizes. I.e., what kind of brightness. You
162 cannot specify more than one of these.
163 The -luminosity option says to use the luminosity (i.e. the 'Y' in the
165 YUV or YCbCr color space) as the pixel's brightness. The luminosity is
166 a measure of how bright a human eye would find the color, taking into
167 account the fact that the human eye is more sensitive to some RGB com‐
168 ponents than others.
169 This option is default.
171 This option is meaningless on grayscale images.
173 Before Netpbm 10.28 (August 2005), there was no -luminosity option, but
175 its meaning was still the default.
176 Before Netpbm 10.28 (August 2005), there was no -colorvalue option.
178 The -colorvalue option says to use the color value (i.e. the 'V' in the
180 HSV color space) as the pixel's brightness. The color value is the
181 gamma-adjusted intensity of the most intense RGB component.
182 This option is meaningless on grayscale images.
184 Before Netpbm 10.28 (August 2005), there was no -colorvalue option.
186 The -saturation option says to use the saturation (i.e. the 'S' in the
188 HSV color space) as the pixel's brightness. The saturation is the
189 ratio of the intensity of the most intense RGB component to the differ‐
190 ence between the intensities of the most and least intense RGB compo‐
191 nent (all intensities gamma-adjusted).
192 In this case, 'brightness' is more of a metaphor than anything.
194 'bright' means saturated and 'dark' means unsaturated.
195 This option is meaningless on grayscale images.
197 Before Netpbm 10.28 (August 2005), there was no -colorvalue option.
199
204 pnmhisteq(1), ppmhist(1), pgmhist(1), pnmgamma(1), ppmbrighten(1),
205 ppmdim(1), pnm(1)
206netpbm documentation 6 January 2006 Pnmnorm User Manual(0)