1\ 2.\" This man page was generated by the Netpbm tool 'makeman' from HTML source. 3.\" Do not hand-hack it! If you have bug fixes or improvements, please find 4.\" the corresponding HTML page on the Netpbm website, generate a patch 5.\" against that, and send it to the Netpbm maintainer. 6.TH "Ppmcie User Manual" 0 "July 31, 2005" "netpbm documentation" 7 8.SH NAME 9 10ppmcie - draw a CIE color chart as a PPM image 11 12.UN synopsis 13.SH SYNOPSIS 14 15 16\fBppmcie\fP 17 18[ 19\fB-rec709\fP|\fB-cie\fP|\fB-ebu\fP|\fB-hdtv\fP|\fB-ntsc\fP|\fB-smpte\fP 20] 21[\fB-xy\fP|\fB-upvp\fP] 22 23[\fB-red\fP \fIrx\fP \fIry\fP] 24 25[\fB-green\fP \fIgx\fP \fIgy\fP] 26 27[\fB-blue\fP \fIbx\fP \fIby\fP] 28 29[\fB-white\fP \fIwx\fP \fIwy\fP] 30 31[\fB-size\fP \fIedge\fP] 32 33[{\fB-xsize\fP|\fB-width\fP} \fIwidth\fP] 34 35[{\fB-ysize\fP|\fB-height\fP} \fIheight\fP] 36 37[\fB-noblack\fP] 38[\fB-nowpoint\fP] 39[\fB-nolabel\fP] 40[\fB-noaxes\fP] 41[\fB-full\fP] 42 43.UN description 44.SH DESCRIPTION 45.PP 46This program is part of 47.BR "Netpbm" (1)\c 48\&. 49 50\fBppmcie\fP creates a PPM file containing a plot of the CIE 51"tongue" color chart -- to the extent possible in a PPM 52image. Alternatively, creates a pseudo-PPM image of the color tongue 53using RGB values from a color system of your choice. 54.PP 55The CIE color tongue is an image of all the hues that can be described 56by CIE X-Y chromaticity coordinates. They are arranged on a two 57dimensional coordinate plane with the X chromaticity on the horizontal 58axis and the Y chromaticity on the vertical scale. (You can choose 59alternatively to use CIE u'-v' chromaticity coordinates, but the 60general idea of the color tongue is the same). 61.PP 62Note that the PPM format specifies that the RGB values in the file are 63from the ITU-R Recommendation BT.709 color system, gamma-corrected. 64And positive. See 65.BR "ppm" (5)\c 66\& for details. If 67you use one of the color system options on \fBppmcie\fP, what you get 68is not a true PPM image, but is very similar. If you display such 69\fBppmcie\fP output using a device that expects PPM input (which 70includes just about any computer graphics display program), it will 71display the wrong colors. 72.PP 73However, you may have a device that expects one of these variations on 74PPM. 75.PP 76In every RGB color system you can specify, including the default 77(which produces a true PPM image) there are hues in the color tongue 78that can't be represented. For example, monochromatic blue-green with 79a wavelength of 500nm cannot be represented in a PPM image. 80.PP 81For these hues, \fBppmcie\fP substitutes a similar hue as follows: 82They are desaturated and rendered as the shade where the edge of the 83Maxwell triangle intersects a line drawn from the requested shade to 84the white point defined by the color system's white point. 85Furthermore, unless you specify the \fB-full\fP option, \fBppmcie\fP 86reduces their intensity by 25% compared to the true hues in the image. 87.PP 88\fBppmcie\fP draws and labels the CIE X-Y coordinate axes unless you 89choose otherwise with options. 90.PP 91\fBppmcie\fP draws the Maxwell triangle for the color system in use 92on the color tongue. The Maxwell triangle is the triangle whose 93vertices are the primary illuminant hues for the color system. The 94hues inside the triangle show the color gamut for the color system. 95They are also the only ones that are correct for the CIE X-Y 96chromaticity coordinates shown. (See explanation above). \fBppmcie\fP 97denotes the Maxwell triangle by rendering it at full brightness, while 98rendering the rest of the color tongue as 3/4 brightness. You can turn 99this off with options. 100.PP 101\fBppmcie\fP also places a black cross at the color system's white 102point (with the center of the cross open so you can actually see the 103white color) and displays in text the CIE X-Y chromaticities of the 104primary illuminants and white point for the color system. You can 105turn this off with options, though. 106.PP 107\fBppmcie\fP annotates the periphery of the color tongue with the 108wavelength, in nanometers of the monochromatic hues which appear 109there. 110.PP 111\fBppmcie\fP displays the black body chromaticity curve for Planckian 112radiators from 1000 to 30000 kelvins on the image. This curve traces the 113colors of black bodies as various temperatures. 114.PP 115You can choose from several standard color systems, or specify one of 116your own numerically. 117.PP 118CIE charts, by their very nature, contain a very large number of 119colors. If you're encoding the chart for a color mapped device or 120file format, you'll need to use \fBpnmquant\fP or \fBppmdither\fP to 121reduce the number of colors in the image. 122 123.UN options 124.SH OPTIONS 125.PP 126You may abbreviate any option to its shortest unique prefix. 127 128 129.TP 130\fB-rec709\fP 131.TP 132\fB-cie\fP 133.TP 134\fB-ebu\fP 135.TP 136\fB-hdtv\fP 137.TP 138\fB-ntsc\fP 139.TP 140\fB-smpte\fP 141Select a standard color system whose gamut to plot. The default is 142\fB-rec709\fP, which chooses ITU-R Recommendation BT.709, 143gamma-corrected. This is the only color system for which 144\fBppmcie\fP's output is a true PPM image. See explanation above. 145\fB-ebu\fP chooses the primaries used in the PAL and SECAM 146broadcasting standards. \fB-ntsc\fP chooses the primaries specified 147by the NTSC broadcasting system (few modern monitors actually cover 148this range). \fB-smpte\fP selects the primaries recommended by the 149Society of Motion Picture and Television Engineers (SMPTE) in 150standards RP-37 and RP-145, and \fB-hdtv\fP uses the much broader 151\fIHDTV ideal\fP primaries. \fB-cie\fP chooses a color system that 152has the largest possible gamut within the spectrum of the chart. This 153is the same color system as you get with the \fB-cie\fP option to 154John Walker's \fBcietoppm\fP program. 155 156.TP 157\fB-xy\fP 158plot CIE 1931 x y chromaticities. This is the default. 159 160.TP 161\fB-upvp\fP 162plot u' v' 1976 chromaticities rather than CIE 1931 x y 163chromaticities. The advantage of u' v' coordinates is that equal 164intervals of distance on the u' v' plane correspond roughly to the 165eye's ability to discriminate colors. 166 167.TP 168\fB-red\fP\fI rx ry\fP 169specifies the CIE \fIx\fP and \fIy\fP co-ordinates of the red 170illuminant of a custom color system and selects the custom system. 171 172.TP 173\fB-green\fP\fI gx gy\fP 174specifies the CIE \fIx\fP and \fIy\fP co-ordinates of the green 175illuminant of the color system and selects the custom system. 176 177.TP 178\fB-blue\fP\fI bx by\fP 179specifies the CIE \fIx\fP and \fIy\fP co-ordinates of the blue 180illuminant of the color system and selects the custom system. 181 182.TP 183\fB-white\fP\fI wx wy\fP 184specifies the CIE \fIx\fP and \fIy\fP co-ordinates of the white 185point of the color system and selects the custom system. 186 187.TP 188\fB-size\fP\fI edge\fP 189Create an image of \fIedge\fP by \fIedge\fP pixels. The default is 190512x512. 191 192.TP 193\fB-xsize|-width\fP\fI width\fP 194Sets the width of the generated image to \fIwidth\fP pixels. The 195default width is 512 pixels. If the height and width of the image are 196not the same, the CIE diagram will be stretched in the longer 197dimension. 198 199.TP 200\fB-ysize|-height\fP\fI height\fP 201Sets the height of the generated image to \fIheight\fP pixels. The 202default height is 512 pixels. If the height and width of the image 203are not the same, the CIE diagram will be stretched in the longer 204dimension. 205 206.TP 207\fB-noblack\fP 208Don't plot the black body chromaticity curve. 209 210.TP 211\fB-nowpoint\fP 212Don't plot the color system's white point. 213 214.TP 215\fB-nolabel\fP 216Omit the label. 217 218.TP 219\fB-noaxes\fP 220Don't plot axes. 221 222.TP 223\fB-full\fP 224Plot the entire CIE tongue in full brightness; don't dim the part 225which is outside the gamut of the specified color system (i.e. outside 226the Maxwell triangle). 227 228 229 230.UN interpret 231.SH INTERPRETATION OF COLOR CHART 232.PP 233A color spectrum is a linear combination of one or more monochromatic 234colors. 235.PP 236A color is a set of color spectra that all look the same to the 237human eye (and brain). Actually, for the purposes of the definition, 238we assume the eye has infinite precision, so we can call two color 239spectra different colors even though they're so close a person 240couldn't possibly tell them apart. 241.PP 242The eye contains 3 kinds of color receptors (cones). Each has a 243different response to the various monochromatic colors. One kind 244responds most strongly to blue, another red, another green. Because 245there are only three, many different color spectra will excite the 246cones at exactly the same level, so the eye cannot tell them apart. 247All such spectra that excite the cones in the same way are a single 248color. 249.PP 250Each point in the color tongue represents a unique color. But 251there are an infinite number of color spectra in the set that is that 252color; i.e. an infinite number of color spectra that would look to you 253like this point. A machine could tell them apart, but you could not. 254.PP 255Remember that the colors outside the highlighted triangle are 256approximations of the real colors because the PPM format cannot 257represent them (and your display device probably cannot display them). 258That is, unless you're using a variation of PPM and a special display 259device, as discussed earlier in this manual. 260.PP 261A color is always relative to some given maximum brightness. A 262particular beam of light looks lime green if in a dim field, but 263pea green if in a bright field. An image on a movie screen may 264look pitch black because the projector is not shining any light on 265it, but when you turn off the projector and look at the same spot in 266room light, the screen looks quite white. The same light from that spot 267hit your eye with the project on as with it off. 268.PP 269The chart shows two dimensions of color. The third is intensity. 270All the colors in the chart have the same intensity. To get all 271possible colors in the gamut, Make copies of the whole chart at every 272intensity between zero and the maximum. 273.PP 274The edge of the tongue consists of all the monochromatic colors. 275A monochromatic color is one with a single wavelength. I.e. a color 276that is in a rainbow. The numbers you see are the wavelengths in 277nanometers. 278.PP 279Any straight line segment within the tongue contains colors which 280are linear combinations of two colors -- the colors at either end of 281the line segment. 282.PP 283Any color in the chart can be created from two other colors (actually, 284from any of an infinite number of pairs of other colors). 285.PP 286All the colors within a triangle inside the tongue can be created 287from a linear combination of the colors at the vertices of that triangle. 288.PP 289Any color in the tongue can be created from at most 3 monochromatic 290colors. 291.PP 292The highlighted triangle shows the colors that can be expressed 293in the tristimulus color system you chose. (ITU-R BT.709 by default). 294The corners of the triangle are the 3 primary illuminants in that 295system (a certain red, green, and blue for BT.709). The edges of 296the triangle, then, represent the colors you can represent with two 297of the primary illuminants (saturated colors), and the interior colors 298require all three primary illuminants (are not saturated). 299.PP 300In the ITU-R BT.709 color system (the default), the white point is 301defined as D65, which is (and is named after) the color of a black 302body at 6502 kelvins. Therefore, you should see the temperature curve 303on the image pass through the white part of the image, and the cross 304that marks the white point, at 6502 kelvins. 305.PP 306D65 white is supposed to be the color of the sun. If you have a 307perfect BT.709 display device, you should see the color of the sun 308at the white point cross. That's an important color, because when you 309look at an object in sunlight, the color that reflects of the object 310is based on the color of sunlight. Note that the sun produces a 311particular color spectrum, but many other color spectra are the same 312color, and display devices never use the actual color spectrum of the 313sun. 314.PP 315The colors at the corners of the triangle have the chromaticities 316phosphors in a monitor that uses the selected color system. Note 317that in BT.709 they are very close to monochromatic red, green, 318and blue, but not quite. That's why you can't display even one true 319color of the rainbow on a video monitor. 320.PP 321Remember that the chart shows colors of constant intensity, 322therefore the corners of the triangles are not the full colors of the 323primary illuminants, but only their chromaticities. In fact, the 324illuminants typically have different intensities. In BT.709, the 325blue primary illuminant is far more intense than the green, which is 326more intense than the red. Designers did this in order to make an 327equal combination of red, green, and blue generate gray. I.e. a 328combination of full strength red, full strength green, and full 329strength blue BT.709 primary illuminants is D65 white. 330.PP 331The tongue has a sharp straight edge at the bottom because that's 332the limit of human vision. There are colors below that line, but they 333involve infrared and ultraviolet light, so you can't see them. This 334line is called the "line of purples." 335 336 337.UN seealso 338.SH SEE ALSO 339.BR "ppmdither" (1)\c 340\&, 341.BR "pnmquant" (1)\c 342\&, 343.BR "ppm" (5)\c 344\& 345 346.UN author 347.SH AUTHOR 348.PP 349Copyright (C) 1995 by John Walker (\fIkelvin@fourmilab.ch\fP) 350.PP 351WWW home page: 352.UR http://www.fourmilab.ch/ 353http://www.fourmilab.ch/ 354.UE 355\& 356.PP 357Permission to use, copy, modify, and distribute this software and its 358documentation for any purpose and without fee is hereby granted, 359without any conditions or restrictions. This software is provided as 360is without express or implied warranty. 361.SH DOCUMENT SOURCE 362This manual page was generated by the Netpbm tool 'makeman' from HTML 363source. The master documentation is at 364.IP 365.B http://netpbm.sourceforge.net/doc/ppmcie.html 366.PP