Section: User Commands (1)
Updated: 26th September 1994
ppmcie - draw a CIE color chart as a PPM image
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.
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.
Copyright (C) 1995 by John Walker (firstname.lastname@example.org)
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.
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