Provided by: netpbm_11.01.00-2build1_amd64 bug

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)  for
       details.  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 display 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  chromaticity  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

       In addition to the options common to all programs based on libnetpbm (most notably -quiet,
       see
        Common Options ⟨index.html#commonoptions⟩ ), ppmcie recognizes the following command line
       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  Recommendation  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 program.

       -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'  coordinates 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 custom 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 dimension.

       -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 purposes 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  monochromatic  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
       illuminants (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 produces 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 (kelvin@fourmilab.ch)

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

DOCUMENT SOURCE

       This manual page was generated by the Netpbm tool 'makeman' from HTML source.  The  master
       documentation is at

              http://netpbm.sourceforge.net/doc/ppmcie.html