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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 chro-
       maticity 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 Rec-
       ommendation 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 ren-
       dered 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.  Further-
       more, 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 chro-
       maticities 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 tem-
       peratures.

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

       CIE charts, by their very nature, contain a very large number of colors.  If you're encod-
       ing 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 Recommendation BT.709, gamma-corrected.  This is the only color  sys-
	      tem  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 moni-
	      tors 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 advan-
	      tage 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 sys-
	      tem 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 num-
       ber of color spectra in the set that is that color; i.e. an infinite number of color spec-
       tra  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 can-
       not 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  combina-
       tion 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  pri-
       mary 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 illumi-
       nants  (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 monochro-
       matic 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  chromatici-
       ties.  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.

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