PPMCIE
Section: User Commands (1)
Updated: 26th September 1994
Index
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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]
All options can be abbreviated to their shortest unique prefix.
DESCRIPTION
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 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 CIE Rec. 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 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
also places a mark at the color system's white point 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.
Finally,
ppmcie
displays the black body chromaticity curve for Planckian radiators from
1000 to 30000 kelvins on the image.
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
colors. If you're encoding the chart for a color mapped device or
file format, you'll need to use
ppmquant
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 CIE Rec. 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 chromaticities.
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 intensity; don't enhance the gamut of
the specified color system.
SEE ALSO
ppmdither(1),
ppmquant(1),
ppm(5)
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,
without any conditions or restrictions. This software is provided ``as
is'' without express or implied warranty.
Index
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- OPTIONS
-
- SEE ALSO
-
- AUTHOR
-
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