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Dark GDK / Drawing Lines

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ReiKumar
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Posted: 11th Oct 2009 01:58
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Yes I know a simple question, but I have a Solar System program where the planets orbit the sun. I want to draw the orbit lines.

The main problem comes in when the player gets to move the planet around to see what the gravitation changes would do. I want to show this by drawing the line of the new orbit.

So, I'm not sure how to draw a variable orbit line in GDK.

Any ideas?
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luke810
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Posted: 11th Oct 2009 17:01
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What do you mean by variable orbit line? You should be able to parameterize the orbit curve and then draw the path using those equations and iterating over small intervals.

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ReiKumar
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Posted: 12th Oct 2009 00:23
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Well I problem that I'm facing is drawing a line that keeps its shape as the user moves around the screen. Also, the dbLine function only draws a straight line from A to B.
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entomophobiac
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Posted: 12th Oct 2009 01:01
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Interesting... What do you use for rotations, ReiKumar? I've had trouble with the actual orbits, myself.
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ReiKumar
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Posted: 13th Oct 2009 01:57
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I don't have anything doing the orbits for me yet, but I will have it once I finish my orbital function, which at the moment can only be tested if I have the orbit drawing function working.
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MxM111
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Posted: 13th Oct 2009 07:47
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The orbit is a parametrized function in 3D space (or 2D space if you have 2D app). This function is something like
x = function1(t);
y = function2(t);
z = function3(t);

where x, y, and z are possible coordinate of planet from parameter t (t actually can be time but not necessarily so). So the orbit is a set of x(t), y(t) z(t).

In the simplest case of circular orbit around point 0,0 in z-plane those functions can be:

x = R*cos(t);
y = R*sin(t);
z = 0;

where R is the orbit radius. You function will be more complex, to account for shifts, rotations and may be elliptical orbits.

What you have to do, in general case, is to run t from 0 to 2pi and draw a short lines using dbLine. Something like
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