Constellations
Constellations
Goals: To become familiar with the constellations data set; altering the transparency and color values for objects and labels.
Before starting, turn on: stars, mwVis, constel
You will be using: alpha, cment, and the textcment commands
Much as we might spot a cloud in the sky and imagine what it resembles, ancient astronomers looked to the stars and dreamed of the heroes and cowards, animals and monsters, or kings and queens of their lore. Given this task today, perhaps we would see star patterns in the form of Gandhi, a computer, or Bill Clinton. The constellations adopted by astronomers include those from ancient Greek times as well as several modern constellations, mostly in the sky as seen from the Southern Hemisphere. For more information on their origin, see “Constellation Connectivity Lines.”
We represent these celestial beings as “stick figures” connecting the bright stars that form their shapes. There are three colors assigned to the constellations in the mw.cf file. Most constellations have a dark pink shade. The zodiacal constellations are bright red and Orion and Ursa Major are given a blue color. The data also have labels associated with them. These labels are placed at an arbitrary distance of 65 light-years.
Brightening the Lines
If you want to brighten or dim the constellation lines, you must use the alpha command. (The slum command that scales the luminosity works only on points.) Use the Alpha Slider (choose alpha from the Slider Menu) to increase or decrease the opaqueness. If alpha = 0, the lines are completely transparent (invisible); if alpha = 1, the lines are opaque. We don't recommend using an alpha of 1, as it will produce unwanted graphics effects. Use alpha = 0.99 if you want really bright lines.
You may also use the Partiview Command Line to set an alpha value. For example, type
alpha 0.5to bring the lines to half transparency.
Changing Their Color
If you're unhappy with the colors we've chosen, you are free to change it. In Partiview, particles are assigned a color set by the color command; however, objects and labels are assigned color using the cment and textcment commands. Well, it's actually more complicated than that. cment assigns a red-green-blue color to a reference number called a color index, and this color index is then assigned to the object.
In the Digital Universe, we try to use color indexes of 1 when possible. Occasionally, a data group has more than one object or more than one color assigned to an object (as is the case for the constellations), making it necessary to use more than one color index. For the constellations, we use a color index of 3 for Orion and Ursa Major, a color index of 2 for the zodiacal constellations, and a color index of 1 for the remaining constellations. Partiview will report the color assigned to color index 1 when you type
cment 1and so on for the remaining color indexes. Remember, the colors are expressed in red, green, blue values, which range from 0-1. A color value of 0 means there is no color present, while a value of 1 is color saturation. For a pure red color we have (R, G, B) = (1, 0, 0). White is (R, G, B) = (1, 1, 1), black is (R, G, B) = (0, 0, 0), and pure yellow is (R, G, B) = (1, 1, 0). To make the lines a nice greenish color, type
cment 1 .2 .7 .2
Changing the Label Color
Similarly, the label color may be changed with the textcment command:
textcment 1 .2 .7 .2You may notice that the labels appear much brighter than the lines. This is because the lines have an alpha channel factored into their display. Bring the alpha up to 0.9 and the lines and labels will resemble one another. This is not the best solution, though, as the lines dominate the view. Better to bring the alpha down so that the lines are subtle, then desaturate the label color. For example, the commands
alpha 0.4will bring the color of the lines and labels into general agreement.
textcment 1 0.05 0.4 0.05
If you like these changes and wish to preserve them, open the mw.cf file and change the values for these commands in the constellation section.
© 2002-2005 American Museum of Natural History
Last Modified: 2007-12-19 by Brian Abbott