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

       ppmcie -	draw a CIE color chart as a PPM	image


       [ -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]

       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	dimen-
       sional coordinate plane with the	X chromaticity on the horizontal  axis
       and the Y chromaticity on the vertical scale.  (You can choose alterna-
       tively 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(5)  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 dis-
       play program), it will display the wrong	colors.

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

       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	radia-
       tors 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  col-
       ors.   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.

       You may abbreviate any option to	its shortest unique prefix.






       -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	 chro-
	      maticities.   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 de-
	      fault 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.

	      Don't plot the black body	chromaticity curve.

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

	      Omit the label.

	      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).

       A  color	 spectrum is a linear combination of one or more monochromatic

       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 as-
       sume the	eye has	infinite precision, so we can call two	color  spectra
       different  colors even though they're so	close a	person couldn't	possi-
       bly tell	them apart.

       The eye contains	3 kinds	of color receptors (cones).  Each has  a  dif-
       ferent 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 ex-
       actly 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  approxi-
       mations of the real colors because the PPM format cannot	represent them
       (and your display device	probably cannot	display	them).	That  is,  un-
       less  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 partic-
       ular 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 be-
       cause  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 nanome-

       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

       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 de-
       fined 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 per-
       fect 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 phos-
       phors 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 il-
       luminants,  but	only  their  chromaticities.  In fact, the illuminants
       typically have different	intensities.  In BT.709, the blue primary  il-
       luminant	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  pri-
       mary 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  in-
       volve infrared and ultraviolet light, so	you can't see them.  This line
       is called the "line of purples."

       ppmdither(1), pnmquant(1), ppm(5)

       Copyright (C) 1995 by John Walker (

       WWW home	page: <>

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

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

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


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