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)fordetails.If you use one of the color system
49 options on ppmcie, what you get is not a true PPM image, but is very
50 similar. If you display such ppmcie output using a device that expects
51 PPM input (which includes just about any computer graphics display pro‐
52 gram), 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 You may abbreviate any option to its shortest unique prefix.
105 -rec709
109 -cie
111 -ebu
113 -hdtv
115 -ntsc
117 -smpte Select a standard color system whose gamut to plot. The default
119 is -rec709, which chooses ITU-R Recommendation BT.709, gamma-
120 corrected. This is the only color system for which ppmcie's
121 output is a true PPM image. See explanation above. -ebu
122 chooses the primaries used in the PAL and SECAM broadcasting
123 standards. -ntsc chooses the primaries specified by the NTSC
124 broadcasting system (few modern monitors actually cover this
125 range). -smpte selects the primaries recommended by the Society
126 of Motion Picture and Television Engineers (SMPTE) in standards
127 RP-37 and RP-145, and -hdtv uses the much broader HDTV ideal
128 primaries. -cie chooses a color system that has the largest
129 possible gamut within the spectrum of the chart. This is the
130 same color system as you get with the -cie option to John
131 Walker's cietoppm program.
132 -xy plot CIE 1931 x y chromaticities. This is the default.
135 -upvp plot u' v' 1976 chromaticities rather than CIE 1931 x y chro‐
138 maticities. The advantage of u' v' coordinates is that equal
139 intervals of distance on the u' v' plane correspond roughly to
140 the eye's ability to discriminate colors.
141 -red rx ry
144 specifies the CIE x and y co-ordinates of the red illuminant of
145 a custom color system and selects the custom system.
146 -green gx gy
149 specifies the CIE x and y co-ordinates of the green illuminant
150 of the color system and selects the custom system.
151 -blue bx by
154 specifies the CIE x and y co-ordinates of the blue illuminant of
155 the color system and selects the custom system.
156 -white wx wy
159 specifies the CIE x and y co-ordinates of the white point of the
160 color system and selects the custom system.
161 -size edge
164 Create an image of edge by edge pixels. The default is 512x512.
165 -xsize|-width width
168 Sets the width of the generated image to width pixels. The
169 default width is 512 pixels. If the height and width of the
170 image are not the same, the CIE diagram will be stretched in the
171 longer dimension.
172 -ysize|-height height
175 Sets the height of the generated image to height pixels. The
176 default height is 512 pixels. If the height and width of the
177 image are not the same, the CIE diagram will be stretched in the
178 longer dimension.
179 -noblack
182 Don't plot the black body chromaticity curve.
183 -nowpoint
186 Don't plot the color system's white point.
187 -nolabel
190 Omit the label.
191 -noaxes
194 Don't plot axes.
195 -full Plot the entire CIE tongue in full brightness; don't dim the
198 part which is outside the gamut of the specified color system
199 (i.e. outside the Maxwell triangle).
200
205 A color spectrum is a linear combination of one or more monochromatic
206 colors.
207 A color is a set of color spectra that all look the same to the human
209 eye (and brain). Actually, for the purposes of the definition, we
210 assume the eye has infinite precision, so we can call two color spectra
211 different colors even though they're so close a person couldn't possi‐
212 bly tell them apart.
213 The eye contains 3 kinds of color receptors (cones). Each has a dif‐
215 ferent response to the various monochromatic colors. One kind responds
216 most strongly to blue, another red, another green. Because there are
217 only three, many different color spectra will excite the cones at
218 exactly the same level, so the eye cannot tell them apart. All such
219 spectra that excite the cones in the same way are a single color.
220 Each point in the color tongue represents a unique color. But there
222 are an infinite number of color spectra in the set that is that color;
223 i.e. an infinite number of color spectra that would look to you like
224 this point. A machine could tell them apart, but you could not.
225 Remember that the colors outside the highlighted triangle are approxi‐
227 mations of the real colors because the PPM format cannot represent them
228 (and your display device probably cannot display them). That is,
229 unless you're using a variation of PPM and a special display device, as
230 discussed earlier in this manual.
231 A color is always relative to some given maximum brightness. A partic‐
233 ular beam of light looks lime green if in a dim field, but pea green if
234 in a bright field. An image on a movie screen may look pitch black
235 because the projector is not shining any light on it, but when you turn
236 off the projector and look at the same spot in room light, the screen
237 looks quite white. The same light from that spot hit your eye with the
238 project on as with it off.
239 The chart shows two dimensions of color. The third is intensity. All
241 the colors in the chart have the same intensity. To get all possible
242 colors in the gamut, Make copies of the whole chart at every intensity
243 between zero and the maximum.
244 The edge of the tongue consists of all the monochromatic colors. A
246 monochromatic color is one with a single wavelength. I.e. a color that
247 is in a rainbow. The numbers you see are the wavelengths in nanome‐
248 ters.
249 Any straight line segment within the tongue contains colors which are
251 linear combinations of two colors -- the colors at either end of the
252 line segment.
253 Any color in the chart can be created from two other colors (actually,
255 from any of an infinite number of pairs of other colors).
256 All the colors within a triangle inside the tongue can be created from
258 a linear combination of the colors at the vertices of that triangle.
259 Any color in the tongue can be created from at most 3 monochromatic
261 colors.
262 The highlighted triangle shows the colors that can be expressed in the
264 tristimulus color system you chose. (ITU-R BT.709 by default). The
265 corners of the triangle are the 3 primary illuminants in that system (a
266 certain red, green, and blue for BT.709). The edges of the triangle,
267 then, represent the colors you can represent with two of the primary
268 illuminants (saturated colors), and the interior colors require all
269 three primary illuminants (are not saturated).
270 In the ITU-R BT.709 color system (the default), the white point is
272 defined as D65, which is (and is named after) the color of a black body
273 at 6502 kelvins. Therefore, you should see the temperature curve on
274 the image pass through the white part of the image, and the cross that
275 marks the white point, at 6502 kelvins.
276 D65 white is supposed to be the color of the sun. If you have a per‐
278 fect BT.709 display device, you should see the color of the sun at the
279 white point cross. That's an important color, because when you look at
280 an object in sunlight, the color that reflects of the object is based
281 on the color of sunlight. Note that the sun produces a particular
282 color spectrum, but many other color spectra are the same color, and
283 display devices never use the actual color spectrum of the sun.
284 The colors at the corners of the triangle have the chromaticities phos‐
286 phors in a monitor that uses the selected color system. Note that in
287 BT.709 they are very close to monochromatic red, green, and blue, but
288 not quite. That's why you can't display even one true color of the
289 rainbow on a video monitor.
290 Remember that the chart shows colors of constant intensity, therefore
292 the corners of the triangles are not the full colors of the primary
293 illuminants, but only their chromaticities. In fact, the illuminants
294 typically have different intensities. In BT.709, the blue primary
295 illuminant is far more intense than the green, which is more intense
296 than the red. Designers did this in order to make an equal combination
297 of red, green, and blue generate gray. I.e. a combination of full
298 strength red, full strength green, and full strength blue BT.709 pri‐
299 mary illuminants is D65 white.
300 The tongue has a sharp straight edge at the bottom because that's the
302 limit of human vision. There are colors below that line, but they
303 involve infrared and ultraviolet light, so you can't see them. This
304 line is called the 'line of purples.'
305
310 ppmdither(1), pnmquant(1), ppm(1)
311
314 Copyright (C) 1995 by John Walker (kelvin@fourmilab.ch)
315 WWW home page: http://www.fourmilab.ch/ ⟨http://www.fourmilab.ch/⟩
317 Permission to use, copy, modify, and distribute this software and its
319 documentation for any purpose and without fee is hereby granted, with‐
320 out any conditions or restrictions. This software is provided as is
321 without express or implied warranty.
322netpbm documentation July 31, 2005 Ppmcie User Manual(0)