1Ppmcie User Manual(0) Ppmcie User Manual(0)
2
6 ppmcie - draw a CIE color chart as a PPM image
7
10 ppmcie
11 [ -rec709|-cie|-ebu|-hdtv|-ntsc|-smpte ] [-xy|-upvp]
13 [-red rx ry]
15 [-green gx gy]
17 [-blue bx by]
19 [-white wx wy]
21 [-size edge]
23 [{-xsize|-width} width]
25 [{-ysize|-height} height]
27 [-noblack] [-nowpoint] [-nolabel] [-noaxes] [-full]
29
32 This program is part of Netpbm(1).
33 ppmcie creates a PPM file containing a plot of the CIE "tongue" color
35 chart -- to the extent possible in a PPM image. Alternatively, creates
36 a pseudo-PPM image of the color tongue using RGB values from a color
37 system of your choice.
38 The CIE color tongue is an image of all the hues that can be described
40 by CIE X-Y chromaticity coordinates. They are arranged on a two dimen‐
41 sional coordinate plane with the X chromaticity on the horizontal axis
42 and the Y chromaticity on the vertical scale. (You can choose alterna‐
43 tively to use CIE u'-v' chromaticity coordinates, but the general idea
44 of the color tongue is the same).
45 Note that the PPM format specifies that the RGB values in the file are
47 from the ITU-R Recommendation BT.709 color system, gamma-corrected.
48 And positive. See ppm(1) for details. If you use one of the color
49 system options on ppmcie, what you get is not a true PPM image, but is
50 very similar. If you display such ppmcie output using a device that
51 expects PPM input (which includes just about any computer graphics dis‐
52 play program), it will display the wrong colors.
53 However, you may have a device that expects one of these variations on
55 PPM.
56 In every RGB color system you can specify, including the default (which
58 produces a true PPM image) there are hues in the color tongue that
59 can't be represented. For example, monochromatic blue-green with a
60 wavelength of 500nm cannot be represented in a PPM image.
61 For these hues, ppmcie substitutes a similar hue as follows: They are
63 desaturated and rendered as the shade where the edge of the Maxwell
64 triangle intersects a line drawn from the requested shade to the white
65 point defined by the color system's white point. Furthermore, unless
66 you specify the -full option, ppmcie reduces their intensity by 25%
67 compared to the true hues in the image.
68 ppmcie draws and labels the CIE X-Y coordinate axes unless you choose
70 otherwise with options.
71 ppmcie draws the Maxwell triangle for the color system in use on the
73 color tongue. The Maxwell triangle is the triangle whose vertices are
74 the primary illuminant hues for the color system. The hues inside the
75 triangle show the color gamut for the color system. They are also the
76 only ones that are correct for the CIE X-Y chromaticity coordinates
77 shown. (See explanation above). ppmcie denotes the Maxwell triangle
78 by rendering it at full brightness, while rendering the rest of the
79 color tongue as 3/4 brightness. You can turn this off with options.
80 ppmcie also places a black cross at the color system's white point
82 (with the center of the cross open so you can actually see the white
83 color) and displays in text the CIE X-Y chromaticities of the primary
84 illuminants and white point for the color system. You can turn this
85 off with options, though.
86 ppmcie annotates the periphery of the color tongue with the wavelength,
88 in nanometers of the monochromatic hues which appear there.
89 ppmcie displays the black body chromaticity curve for Planckian radia‐
91 tors from 1000 to 30000 kelvins on the image. This curve traces the
92 colors of black bodies as various temperatures.
93 You can choose from several standard color systems, or specify one of
95 your own numerically.
96 CIE charts, by their very nature, contain a very large number of col‐
98 ors. If you're encoding the chart for a color mapped device or file
99 format, you'll need to use pnmquant or ppmdither to reduce the number
100 of colors in the image.
101
104 In addition to the options common to all programs based on libnetpbm
105 (most notably -quiet, see
106 Common Options ⟨index.html#commonoptions⟩ ), ppmcie recognizes the
107 following command line options:
108 You may abbreviate any option to its shortest unique prefix.
110 -rec709
114 -cie
116 -ebu
118 -hdtv
120 -ntsc
122 -smpte Select a standard color system whose gamut to plot. The default
124 is -rec709, which chooses ITU-R Recommendation BT.709, gamma-
125 corrected. This is the only color system for which ppmcie's
126 output is a true PPM image. See explanation above. -ebu
127 chooses the primaries used in the PAL and SECAM broadcasting
128 standards. -ntsc chooses the primaries specified by the NTSC
129 broadcasting system (few modern monitors actually cover this
130 range). -smpte selects the primaries recommended by the Society
131 of Motion Picture and Television Engineers (SMPTE) in standards
132 RP-37 and RP-145, and -hdtv uses the much broader HDTV ideal
133 primaries. -cie chooses a color system that has the largest
134 possible gamut within the spectrum of the chart. This is the
135 same color system as you get with the -cie option to John
136 Walker's cietoppm program.
137 -xy plot CIE 1931 x y chromaticities. This is the default.
140 -upvp plot u' v' 1976 chromaticities rather than CIE 1931 x y chro‐
143 maticities. The advantage of u' v' coordinates is that equal
144 intervals of distance on the u' v' plane correspond roughly to
145 the eye's ability to discriminate colors.
146 -red rx ry
149 specifies the CIE x and y co-ordinates of the red illuminant of
150 a custom color system and selects the custom system.
151 -green gx gy
154 specifies the CIE x and y co-ordinates of the green illuminant
155 of the color system and selects the custom system.
156 -blue bx by
159 specifies the CIE x and y co-ordinates of the blue illuminant of
160 the color system and selects the custom system.
161 -white wx wy
164 specifies the CIE x and y co-ordinates of the white point of the
165 color system and selects the custom system.
166 -size edge
169 Create an image of edge by edge pixels. The default is 512x512.
170 -xsize|-width width
173 Sets the width of the generated image to width pixels. The
174 default width is 512 pixels. If the height and width of the
175 image are not the same, the CIE diagram will be stretched in the
176 longer dimension.
177 -ysize|-height height
180 Sets the height of the generated image to height pixels. The
181 default height is 512 pixels. If the height and width of the
182 image are not the same, the CIE diagram will be stretched in the
183 longer dimension.
184 -noblack
187 Don't plot the black body chromaticity curve.
188 -nowpoint
191 Don't plot the color system's white point.
192 -nolabel
195 Omit the label.
196 -noaxes
199 Don't plot axes.
200 -full Plot the entire CIE tongue in full brightness; don't dim the
203 part which is outside the gamut of the specified color system
204 (i.e. outside the Maxwell triangle).
205
210 A color spectrum is a linear combination of one or more monochromatic
211 colors.
212 A color is a set of color spectra that all look the same to the human
214 eye (and brain). Actually, for the purposes of the definition, we
215 assume the eye has infinite precision, so we can call two color spectra
216 different colors even though they're so close a person couldn't possi‐
217 bly tell them apart.
218 The eye contains 3 kinds of color receptors (cones). Each has a dif‐
220 ferent response to the various monochromatic colors. One kind responds
221 most strongly to blue, another red, another green. Because there are
222 only three, many different color spectra will excite the cones at
223 exactly the same level, so the eye cannot tell them apart. All such
224 spectra that excite the cones in the same way are a single color.
225 Each point in the color tongue represents a unique color. But there
227 are an infinite number of color spectra in the set that is that color;
228 i.e. an infinite number of color spectra that would look to you like
229 this point. A machine could tell them apart, but you could not.
230 Remember that the colors outside the highlighted triangle are approxi‐
232 mations of the real colors because the PPM format cannot represent them
233 (and your display device probably cannot display them). That is,
234 unless you're using a variation of PPM and a special display device, as
235 discussed earlier in this manual.
236 A color is always relative to some given maximum brightness. A partic‐
238 ular beam of light looks lime green if in a dim field, but pea green if
239 in a bright field. An image on a movie screen may look pitch black
240 because the projector is not shining any light on it, but when you turn
241 off the projector and look at the same spot in room light, the screen
242 looks quite white. The same light from that spot hit your eye with the
243 project on as with it off.
244 The chart shows two dimensions of color. The third is intensity. All
246 the colors in the chart have the same intensity. To get all possible
247 colors in the gamut, Make copies of the whole chart at every intensity
248 between zero and the maximum.
249 The edge of the tongue consists of all the monochromatic colors. A
251 monochromatic color is one with a single wavelength. I.e. a color that
252 is in a rainbow. The numbers you see are the wavelengths in nanome‐
253 ters.
254 Any straight line segment within the tongue contains colors which are
256 linear combinations of two colors -- the colors at either end of the
257 line segment.
258 Any color in the chart can be created from two other colors (actually,
260 from any of an infinite number of pairs of other colors).
261 All the colors within a triangle inside the tongue can be created from
263 a linear combination of the colors at the vertices of that triangle.
264 Any color in the tongue can be created from at most 3 monochromatic
266 colors.
267 The highlighted triangle shows the colors that can be expressed in the
269 tristimulus color system you chose. (ITU-R BT.709 by default). The
270 corners of the triangle are the 3 primary illuminants in that system (a
271 certain red, green, and blue for BT.709). The edges of the triangle,
272 then, represent the colors you can represent with two of the primary
273 illuminants (saturated colors), and the interior colors require all
274 three primary illuminants (are not saturated).
275 In the ITU-R BT.709 color system (the default), the white point is
277 defined as D65, which is (and is named after) the color of a black body
278 at 6502 kelvins. Therefore, you should see the temperature curve on
279 the image pass through the white part of the image, and the cross that
280 marks the white point, at 6502 kelvins.
281 D65 white is supposed to be the color of the sun. If you have a per‐
283 fect BT.709 display device, you should see the color of the sun at the
284 white point cross. That's an important color, because when you look at
285 an object in sunlight, the color that reflects of the object is based
286 on the color of sunlight. Note that the sun produces a particular
287 color spectrum, but many other color spectra are the same color, and
288 display devices never use the actual color spectrum of the sun.
289 The colors at the corners of the triangle have the chromaticities phos‐
291 phors in a monitor that uses the selected color system. Note that in
292 BT.709 they are very close to monochromatic red, green, and blue, but
293 not quite. That's why you can't display even one true color of the
294 rainbow on a video monitor.
295 Remember that the chart shows colors of constant intensity, therefore
297 the corners of the triangles are not the full colors of the primary
298 illuminants, but only their chromaticities. In fact, the illuminants
299 typically have different intensities. In BT.709, the blue primary
300 illuminant is far more intense than the green, which is more intense
301 than the red. Designers did this in order to make an equal combination
302 of red, green, and blue generate gray. I.e. a combination of full
303 strength red, full strength green, and full strength blue BT.709 pri‐
304 mary illuminants is D65 white.
305 The tongue has a sharp straight edge at the bottom because that's the
307 limit of human vision. There are colors below that line, but they
308 involve infrared and ultraviolet light, so you can't see them. This
309 line is called the "line of purples."
310
313 ppmdither(1), pnmquant(1), ppm(1)
314
317 Copyright (C) 1995 by John Walker (kelvin@fourmilab.ch)
318 WWW home page: http://www.fourmilab.ch/ ⟨http://www.fourmilab.ch/⟩
320 Permission to use, copy, modify, and distribute this software and its
322 documentation for any purpose and without fee is hereby granted, with‐
323 out any conditions or restrictions. This software is provided as is
324 without express or implied warranty.
325
327 This manual page was generated by the Netpbm tool 'makeman' from HTML
328 source. The master documentation is at
329 http://netpbm.sourceforge.net/doc/ppmcie.html
331netpbm documentation 31 July 2005 Ppmcie User Manual(0)