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]
You may abbreviate any option to its shortest unique prefix.
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 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(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 pro-
gram), 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 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
-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 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 a pixmap 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 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
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 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
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 nanome-
ters.
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 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
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 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
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/
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)
