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.

ppmcie creates a PPM file containing a plot of the CIE 'tongue' color
chart -- to the extent possible in a PPM image. Alternatively, cre-
ates 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 gen-
eral 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 ppmfordetails.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 wave-
length, 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.


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


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


-upvp plot u' v' 1976 chromaticities rather than CIE 1931 x y chro-


-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 Plot the entire CIE tongue in full brightness; don't dim the




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 spec-
tra 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 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 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 approxi-
mations 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 par-
ticular 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 trian-
gle, then, represent the colors you can represent with two of the pri-
mary 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 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 particu-
lar 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 combina-
tion 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, pnmquant, ppm


AUTHOR
Copyright (C) 1995 by John Walker (kelvin@fourmilab.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, with-
out any conditions or restrictions. This software is provided as is
without express or implied warranty.



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