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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 dis-
play 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 AT 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
netpbm documentation July 31, 2005 Ppmcie User Manual(0)
Generated by $Id: phpMan.php,v 4.55 2007/09/05 04:42:51 chedong Exp $ Author: Che Dong
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