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Ppmcie User Manual(0)                                    Ppmcie User Manual(0)



NAME
       ppmcie - draw a CIE color chart as a PPM image


SYNOPSIS
       ppmcie

       [ -rec709|-cie|-ebu|-hdtv|-ntsc|-smpte ] [-xy|-upvp]

       [-red rx ry]

       [-green gx gy]

       [-blue bx by]

       [-white wx wy]

       [-size edge]

       [{-xsize|-width} width]

       [{-ysize|-height} height]

       [-noblack] [-nowpoint] [-nolabel] [-noaxes] [-full]


DESCRIPTION
       This program is part of Netpbm(1).

       ppmcie  creates a PPM file containing a plot of the CIE 'tongue' color chart -- to the extent possible in a PPM
       image.  Alternatively, creates a pseudo-PPM image of the color tongue using RGB values from a color  system  of
       your choice.

       The  CIE  color  tongue  is an image of all the hues that can be described by CIE X-Y chromaticity coordinates.
       They are arranged on a two dimensional coordinate plane with the X chromaticity on the horizontal axis and  the
       Y chromaticity on the vertical scale.  (You can choose alternatively to use CIE u'-v' chromaticity coordinates,
       but the general idea of the color tongue is the same).

       Note that the PPM format specifies that the RGB values in the file are from  the  ITU-R  Recommendation  BT.709
       color  system, gamma-corrected.  And positive.  See ppm(1)fordetails.If you use one of the color system options
       on ppmcie, what you get is not a true PPM image, but is very similar.  If you display such ppmcie output  using
       a device that expects PPM input (which includes just about any computer graphics display program), it will dis-
       play the wrong colors.

       However, you may have a device that expects one of these variations on PPM.

       In every RGB color system you can specify, including the default (which produces a true PPM  image)  there  are
       hues in the color tongue that can't be represented.  For example, monochromatic blue-green with a wavelength of
       500nm cannot be represented in a PPM image.

       For these hues, ppmcie substitutes a similar hue as follows: They are desaturated and  rendered  as  the  shade
       where  the  edge  of  the  Maxwell triangle intersects a line drawn from the requested shade to the white point
       defined by the color system's white point.  Furthermore, unless you specify the -full  option,  ppmcie  reduces
       their intensity by 25% compared to the true hues in the image.

       ppmcie draws and labels the CIE X-Y coordinate axes unless you choose otherwise with options.

       ppmcie draws the Maxwell triangle for the color system in use on the color tongue.  The Maxwell triangle is the
       triangle whose vertices are the primary illuminant hues for the color system.  The  hues  inside  the  triangle
       show  the color gamut for the color system.  They are also the only ones that are correct for the CIE X-Y chro-
       maticity coordinates shown.  (See explanation above).  ppmcie denotes the Maxwell triangle by rendering  it  at
       full  brightness,  while  rendering the rest of the color tongue as 3/4 brightness.  You can turn this off with
       options.

       ppmcie also places a black cross at the color system's white point (with the center of the cross  open  so  you
       can  actually  see  the white color) and displays in text the CIE X-Y chromaticities of the primary illuminants
       and white point for the color system.  You can turn this off with options, though.

       ppmcie annotates the periphery of the color tongue with the wavelength, in nanometers of the monochromatic hues
       which appear there.

       ppmcie  displays  the  black  body chromaticity curve for Planckian radiators from 1000 to 30000 kelvins on the
       image.  This curve traces the colors of black bodies as various temperatures.

       You can choose from several standard color systems, or specify one of your own numerically.

       CIE charts, by their very nature, contain a very large number of colors.  If you're encoding the  chart  for  a
       color  mapped device or file format, you'll need to use pnmquant or ppmdither to reduce the number of colors in
       the image.


OPTIONS
       You may abbreviate any option to its shortest unique prefix.



       -rec709

       -cie

       -ebu

       -hdtv

       -ntsc

       -smpte Select a standard color system whose gamut to plot.  The default is -rec709, which chooses ITU-R  Recom-
              mendation  BT.709,  gamma-corrected.   This is the only color system for which ppmcie's output is a true
              PPM image.  See explanation above.  -ebu chooses the primaries used in the PAL  and  SECAM  broadcasting
              standards.   -ntsc  chooses the primaries specified by the NTSC broadcasting system (few modern monitors
              actually cover this range).  -smpte selects the primaries recommended by the Society of  Motion  Picture
              and  Television  Engineers  (SMPTE)  in standards RP-37 and RP-145, and -hdtv uses the much broader HDTV
              ideal primaries.  -cie chooses a color system that has the largest possible gamut within the spectrum of
              the chart.  This is the same color system as you get with the -cie option to John Walker's cietoppm pro-
              gram.


       -xy    plot CIE 1931 x y chromaticities.  This is the default.


       -upvp  plot u' v' 1976 chromaticities rather than CIE 1931 x y chromaticities.  The advantage of u' v'  coordi-
              nates  is that equal intervals of distance on the u' v' plane correspond roughly to the eye's ability to
              discriminate colors.


       -red rx ry
              specifies the CIE x and y co-ordinates of the red illuminant of a custom color system  and  selects  the
              custom system.


       -green gx gy
              specifies  the CIE x and y co-ordinates of the green illuminant of the color system and selects the cus-
              tom system.


       -blue bx by
              specifies the CIE x and y co-ordinates of the blue illuminant of the color system and selects the custom
              system.


       -white wx wy
              specifies  the  CIE  x  and y co-ordinates of the white point of the color system and selects the custom
              system.


       -size edge
              Create an image of edge by edge pixels.  The default is 512x512.


       -xsize|-width width
              Sets the width of the generated image to width pixels.  The default width is 512 pixels.  If the  height
              and width of the image are not the same, the CIE diagram will be stretched in the longer dimension.


       -ysize|-height height
              Sets  the  height  of  the  generated image to height pixels.  The default height is 512 pixels.  If the
              height and width of the image are not the same, the CIE diagram will be stretched in the  longer  dimen-
              sion.


       -noblack
              Don't plot the black body chromaticity curve.


       -nowpoint
              Don't plot the color system's white point.


       -nolabel
              Omit the label.


       -noaxes
              Don't plot axes.


       -full  Plot  the  entire  CIE  tongue  in full brightness; don't dim the part which is outside the gamut of the
              specified color system (i.e. outside the Maxwell triangle).




INTERPRETATION OF COLOR CHART
       A color spectrum is a linear combination of one or more monochromatic colors.

       A color is a set of color spectra that all look the same to the human eye (and brain).  Actually, for the  pur-
       poses  of  the definition, we assume the eye has infinite precision, so we can call two color spectra different
       colors even though they're so close a person couldn't possibly tell them apart.

       The eye contains 3 kinds of color receptors (cones).  Each has a different response to  the  various  monochro-
       matic  colors.   One  kind  responds most strongly to blue, another red, another green.  Because there are only
       three, many different color spectra will excite the cones at exactly the same level, so  the  eye  cannot  tell
       them apart.  All such spectra that excite the cones in the same way are a single color.

       Each point in the color tongue represents a unique color.  But there are an infinite number of color spectra in
       the set that is that color; i.e. an infinite number of color spectra that would look to you like this point.  A
       machine could tell them apart, but you could not.

       Remember that the colors outside the highlighted triangle are approximations of the real colors because the PPM
       format cannot represent them (and your display device probably cannot display them).  That  is,  unless  you're
       using a variation of PPM and a special display device, as discussed earlier in this manual.

       A color is always relative to some given maximum brightness.  A particular beam of light looks lime green if in
       a dim field, but pea green if in a bright field.  An image on a movie screen may look pitch black  because  the
       projector is not shining any light on it, but when you turn off the projector and look at the same spot in room
       light, the screen looks quite white.  The same light from that spot hit your eye with the project on as with it
       off.

       The  chart  shows  two dimensions of color.  The third is intensity.  All the colors in the chart have the same
       intensity.  To get all possible colors in the gamut, Make copies of the whole chart at every intensity  between
       zero and the maximum.

       The  edge  of  the tongue consists of all the monochromatic colors.  A monochromatic color is one with a single
       wavelength.  I.e. a color that is in a rainbow.  The numbers you see are the wavelengths in nanometers.

       Any straight line segment within the tongue contains colors which are linear combinations of two colors --  the
       colors at either end of the line segment.

       Any  color in the chart can be created from two other colors (actually, from any of an infinite number of pairs
       of other colors).

       All the colors within a triangle inside the tongue can be created from a linear combination of  the  colors  at
       the vertices of that triangle.

       Any color in the tongue can be created from at most 3 monochromatic colors.

       The  highlighted  triangle  shows  the  colors that can be expressed in the tristimulus color system you chose.
       (ITU-R BT.709 by default).  The corners of the triangle are the 3 primary illuminants in that system (a certain
       red, green, and blue for BT.709).  The edges of the triangle, then, represent the colors you can represent with
       two of the primary illuminants (saturated colors), and the interior colors require all  three  primary  illumi-
       nants (are not saturated).

       In  the  ITU-R  BT.709  color  system  (the default), the white point is defined as D65, which is (and is named
       after) the color of a black body at 6502 kelvins.  Therefore, you should see the temperature curve on the image
       pass through the white part of the image, and the cross that marks the white point, at 6502 kelvins.

       D65  white is supposed to be the color of the sun.  If you have a perfect BT.709 display device, you should see
       the color of the sun at the white point cross.  That's an important color, because when you look at  an  object
       in  sunlight,  the color that reflects of the object is based on the color of sunlight.  Note that the sun pro-
       duces a particular color spectrum, but many other color spectra are the same color, and display  devices  never
       use the actual color spectrum of the sun.

       The colors at the corners of the triangle have the chromaticities phosphors in a monitor that uses the selected
       color system.  Note that in BT.709 they are very close to monochromatic red, green, and blue,  but  not  quite.
       That's why you can't display even one true color of the rainbow on a video monitor.

       Remember  that the chart shows colors of constant intensity, therefore the corners of the triangles are not the
       full colors of the primary illuminants, but only their chromaticities.  In fact, the illuminants typically have
       different  intensities.   In  BT.709,  the blue primary illuminant is far more intense than the green, which is
       more intense than the red.  Designers did this in order to make an equal combination of red,  green,  and  blue
       generate  gray.   I.e.   a combination of full strength red, full strength green, and full strength blue BT.709
       primary illuminants is D65 white.

       The tongue has a sharp straight edge at the bottom because that's the limit of human vision.  There are  colors
       below  that  line, but they involve infrared and ultraviolet light, so you can't see them.  This line is called
       the 'line of purples.'




SEE ALSO
       ppmdither(1), pnmquant(1), ppm(1)


AUTHOR
       Copyright (C) 1995 by John Walker (kelvinATfourmilab.ch)

       WWW home page: http://www.fourmilab.ch/

       Permission to use, copy, modify, and distribute this software and its documentation for any purpose and without
       fee is hereby granted, without any conditions or restrictions.  This software is provided as is without express
       or implied warranty.



netpbm documentation             July 31, 2005           Ppmcie User Manual(0)