1Ppmcie User Manual(0)                                    Ppmcie User Manual(0)
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NAME

6       ppmcie - draw a CIE color chart as a PPM image
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SYNOPSIS

10       ppmcie
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12       [ -rec709|-cie|-ebu|-hdtv|-ntsc|-smpte ] [-xy|-upvp]
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14       [-red rx ry]
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16       [-green gx gy]
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18       [-blue bx by]
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20       [-white wx wy]
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22       [-size edge]
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24       [{-xsize|-width} width]
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26       [{-ysize|-height} height]
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28       [-noblack] [-nowpoint] [-nolabel] [-noaxes] [-full]
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30

DESCRIPTION

32       This program is part of Netpbm(1).
33
34       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
39       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
46       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.
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54       However,  you may have a device that expects one of these variations on
55       PPM.
56
57       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.
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62       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
69       ppmcie draws and labels the CIE X-Y coordinate axes unless  you  choose
70       otherwise with options.
71
72       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.
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81       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
87       ppmcie annotates the periphery of the color tongue with the wavelength,
88       in nanometers of the monochromatic hues which appear there.
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90       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.
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94       You  can  choose from several standard color systems, or specify one of
95       your own numerically.
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97       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.
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102

OPTIONS

104       You may abbreviate any option to its shortest unique prefix.
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107
108       -rec709
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110       -cie
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112       -ebu
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114       -hdtv
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116       -ntsc
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118       -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.
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133
134       -xy    plot CIE 1931 x y chromaticities.  This is the default.
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136
137       -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.
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142
143       -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.
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147
148       -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.
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153       -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.
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158       -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.
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162
163       -size edge
164              Create an image of edge by edge pixels.  The default is 512x512.
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167       -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.
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173
174       -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.
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180
181       -noblack
182              Don't plot the black body chromaticity curve.
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185       -nowpoint
186              Don't plot the color system's white point.
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189       -nolabel
190              Omit the label.
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193       -noaxes
194              Don't plot axes.
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196
197       -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).
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203

INTERPRETATION OF COLOR CHART

205       A  color  spectrum is a linear combination of one or more monochromatic
206       colors.
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208       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.
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214       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
221       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.
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226       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
232       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
240       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
245       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
250       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.
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254       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).
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257       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.
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260       Any color in the tongue can be created from  at  most  3  monochromatic
261       colors.
262
263       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
271       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
277       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.
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285       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.
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291       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
301       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."
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SEE ALSO

309       ppmdither(1), pnmquant(1), ppm(1)
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AUTHOR

313       Copyright (C) 1995 by John Walker (kelvin@fourmilab.ch)
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315       WWW home page: http://www.fourmilab.ch/http://www.fourmilab.ch/
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317       Permission to use, copy, modify, and distribute this software  and  its
318       documentation  for any purpose and without fee is hereby granted, with‐
319       out any conditions or restrictions.  This software is  provided  as  is
320       without express or implied warranty.
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DOCUMENT SOURCE

323       This  manual  page was generated by the Netpbm tool 'makeman' from HTML
324       source.  The master documentation is at
325
326              http://netpbm.sourceforge.net/doc/ppmcie.html
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328netpbm documentation             July 31, 2005           Ppmcie User Manual(0)
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