1Pnmgamma User Manual(0) Pnmgamma User Manual(0)
2
6 pnmgamma - perform gamma adjustment on a PNM image
7
10 pnmgamma {
11 -bt709tolinear |
12 -lineartobt709 |
13 -bt709tosrgb |
14 -srgbtobt709 } [-gamma=float] [-rgamma=float] [-ggamma=float]
15 [-bgamma=float]
16 [pnmfile]
18 pnmgamma [
20 -bt709ramp |
21 -srgbramp ] [-ungamma] [{gamma | redgamma greengamma bluegamma} [pnm‐
22 file]]
23
27 This program is part of Netpbm(1).
28 Pnmgamma performs gamma adjustment on pseudo-PNM images.
30 The PPM format specification specifies that certain sample values in a
32 file represent certain light intensities in an image. In particular,
33 they specify that the sample values are directly proportional to lumi‐
34 nance as defined by ITU-R Recommendation BT.709. BT.709 luminance as a
35 function of radiance is a power function modified with a linear ramp
36 near black.
37 However, people sometimes work with approximations of PPM and PGM where
39 the sample values represent intensity in different ways:
40 In one common variation, the sample value is directly proportional to
42 radiance (often called 'linear intensity').
43 Another popular variation is to make the samples proportional to lumi‐
45 nance as defined by the Internation Electrotechnical Commission (IEC)
46 SRGB standard. The SRGB gamma transfer function is like the BT.709 one
47 except with different constants in it.
48 Note that SRGB is often spelled 'sRGB'. In this document, we use stan‐
50 dard English typography, though, which doesn't allow for that kind of
51 capitalization.
52 pnmgamma allows you to manipulate the gamma transfer function, thus
54 working with and/or creating pseudo-PPM files that are useful for vari‐
55 ous things.
56 For example, if you feed a true PPM to pnmgamma -bt709tolinear , you
58 get as output a file which is PPM in every respect except that the sam‐
59 ple values are radiances. If you feed such a file to pnmgamma -lin‐
60 earto709, you get back a true PPM.
61 The situation for PGM images is analogous. And pnmgamma treats PBM
63 images as PGM images.
64 When you feed a radiance-proportional pseudo-PPM image to a display
66 program that expects a true PPM, the display appears darker than it
67 should, so pnmgamma has the effect of lightening the image. When you
68 feed a true PPM to a display program that expects radiance-proportional
69 sample values, and therefore does a gamma adjustment of its own on
70 them, the display appears lighter than it should, so pnmgamma with a
71 gamma value less than one (the multiplicative inverse of whatever gamma
72 value the display program uses) has the effect of darkening the image.
73
76 The form of the parameters depends on whether you're using the old syn‐
77 tax or the new syntax. With the old syntax, the parameters are a mix‐
78 ture of gamma values and the input file name. With the new syntax, the
79 only parameter is the input file name and you specify gamma values with
80 option.
81 You use the old syntax if you specify -bt709ramp (or its synonym -cier‐
83 amp) or -srgramp or if you don't specify any transfer function at all
84 (and thus default to a simple exponential). Otherwise, you use the new
85 syntax.
86 With the old syntax, you may specify a single gamma value or 3 separate
88 gamma values (red, green, and blue) or no gamma values. In any case,
89 the meanings of those parameters is the same as the more modern -gamma,
90 -rgamma, -ggamma, and -bgamma options described below.
91
95 -bt709tolinear
96 Convert the image from BT.709 luminance to radiance. I.e. con‐
97 vert from true PPM or PGM to a radiance-linear variation that
98 can be used with certain tools that need it.
99 This option was new in Netpbm 10.32 (February 2006).
101 -lineartobt709
104 Convert the image from radiance to BT.709 luminance. I.e. con‐
105 vert to true PPM or PGM from a radiance-linear variation.
106 You get true BT.709 (ergo true PPM or PGM) only if you use the
108 default gamma value (i.e. don't specify -gamma, etc.).
109 This option was new in Netpbm 10.32 (February 2006).
111 -bt709tosrgb
114 Convert the image from BT.709 luminance to SRGB luminance. I.e.
115 convert from true PPM or PGM to an SRGB-based variation that is
116 required by certain tools and display devices.
117 You get true SRGB only if you use the default gamma value (i.e.
119 don't specify -gamma, etc.).
120 This option was new in Netpbm 10.32 (February 2006).
122 -srgbtobt709
125 Convert the image from SRGB luminance to BT.709 luminance. I.e.
126 convert to true PPM or PGM from an SRGB-based variation.
127 This option was new in Netpbm 10.32 (February 2006).
129 -bt709ramp
132 Same as -lineartobt709, but using the old syntax.
133 This option was renamed in Netpbm 10.32 (February 2006). Before
135 that, its name is -cieramp.
136 -cieramp
139 This is an obsolete synonym for -bt709ramp.
140 The name of this option comes from a former belief that this was
142 a standard of CIE (International Commission On Illumination),
143 but it now (August 2005) looks like it never was.
144 -srgbramp
147 Convert the image from radiance to SRGB luminance. Note that it
148 is true SRGB only if you use the default gamma value (i.e. don't
149 specify any gamma parameters).
150 This is an old syntax option. There is no equivalent in the new
152 syntax because it really shouldn't be a function of pnmgamma at
153 all. It exists solely for backward compatibility. The reason
154 it shouldn't exist is that the way to do this conversion consis‐
155 tent with the Netpbm philosophy is do a -lineartobt709 followed
156 by a -bt709tosrgb. It's exactly analogous to the way you have
157 to convert from PNG to TIFF by doing a pngtopam followed by a
158 pnmtotiff. The -srgbramp option actually dates to before there
159 was a standard definition of what the sample values of a Netpbm
160 image measure, and pnmgamma considered radiance-linear to be the
161 proper intermediate format.
162 -ungamma
165 Apply the inverse of the specified transfer function (i.e. go
166 from gamma-adjusted luminance to radiance).
167 This is valid only with -bt709ramp (aka -cieramp), -srgbramp,
169 and the default exponential transfer function.
170 -gamma=float
173 This specifies the gamma value to use in the transfer function.
174 All of the transfer functions involve an exponent, and the gamma
175 value is that exponent.
176 The standards specify a particular gamma value. If you use any‐
178 thing else, you are varying from the standard.
179 The default is the standard value. For the simple exponential
181 transfer function (which is not a standard), the default is 2.2.
182 In the -bt709tosrgb and -srgbtobt709 conversions there are two
184 exponents. -gamma affects the 'to' function; the 'from' func‐
185 tion always uses the standard gamma value.
186 If you specify one of the component-specific options (-rgamma,
188 etc.), that overrides the -gamma value.
189 With the -bt709ramp (aka -cieramp), -srgbramp, or the default
191 exponential transfer function, you can't actually use this
192 option, but you specify the same thing with parameters.
193 ⟨#parameters⟩
194 This option was new in Netpbm 10.32 (February 2006).
196 -rgamma=float
199 -ggamma=float
201 -bgamma=float
203 These options are just like -gamma, except they specify the
204 value for a particular one of the color components.
205 If you don't specify this option for a particular color compo‐
207 nent, the default is the -gamma value (or -gamma's default if
208 you didn't specify that either).
209 With the -bt709ramp (aka -cieramp), -srgbramp, or the default
211 exponential transfer function, you can't actually use this
212 option, but you specify the same thing with parameters.
213 ⟨#parameters⟩
214 This option was new in Netpbm 10.32 (February 2006).
216 -maxval=maxval
219 This is the maxval of the output image. By default, the maxval
220 of the output is the same as that of the input.
221 Because the transformation is not linear, you need a greater
223 maxval in the output in order not to lose any information from
224 the input. For example, if you convert to radiance-linear sam‐
225 ple values with with -ungamma -bt709ramp and default gamma
226 value, and your maxval is 255 on both input and output, 3 dif‐
227 ferent input sample values all generate output sample value 254.
228 In order to have a different output sample value for each input
229 sample value, you would need an output maxval at least 3 times
230 the input maxval.
231 This option was new in Netpbm 10.32 (February 2006). Before
233 that, you can achieve the same result by increasing the maxval
234 of the input or decreasing the maxval of the output using
235 pamdepth.
236
241 A good explanation of gamma is in Charles Poynton's Gamma FAQ at
242 http://www.poynton.com/GammaFAQ.html (1) and Color FAQ at
243 http://www.poynton.com/ColorFAQ.html (1).
244 In brief: The simplest way to code an image is by using sample values
246 that are directly proportional to the radiance of the color components.
247 Radiance is a physical quantification based on the amount of power in
248 the light; it is easily measurable in a laboratory, but does not take
249 into account what the light looks like to a person. It wastes the sam‐
250 ple space because the human eye can't discern differences between low-
251 radiance colors as well as it can between high-radiance colors. So
252 instead, we pass the radiance values through a transfer function that
253 makes it so that changing a sample value by 1 causes the same level of
254 perceived color change anywhere in the sample range. We store those
255 resulting values in the image file. That transfer function is called
256 the gamma transfer function and the transformation is called gamma
257 adjusting.
258 The gamma-adjusted value, proportional to subjective brightness, are
260 known as the luminance of the pixel.
261 There is no precise objective way to measure luminance, since it's psy‐
263 chological. Also, perception of brightness varies according to a vari‐
264 ety of factors, including the surrounding in which an image is viewed.
265 Therefore, there is not just one gamma transfer function.
266 Virtually all image formats, either specified or de facto, use gamma-
268 adjusted values for their sample values.
269 What's really nice about gamma is that by coincidence, the inverse
271 function that you have to do to convert the gamma-adjusted values back
272 to radiance is done automatically by CRTs. You just apply a voltage to
273 the CRT's electron gun that is proportional to the gamma-adjusted sam‐
274 ple value, and the radiance of the light that comes out of the screen
275 is close to the radiance value you had before you applied the gamma
276 transfer function!
277 And when you consider that computer video devices usually want you to
279 store in video memory a value proportional to the signal voltage you
280 want to go to the monitor, which the monitor turns into a proportional
281 drive voltage on the electron gun, it is really convenient to work with
282 gamma-adjusted sample values.
283
286 pnm(1)
287
290 Copyright (C) 1991 by Bill Davidson and Jef Poskanzer.
291netpbm documentation 30 June 2007 Pnmgamma User Manual(0)