Bodies attracting bodies

[jedd.jones]

I'm looking for a good way to make one body attract other bodies like a gravitational field. I am wondering if there is some built in way to do this or am I going to need check proximity and do impulses per body?

-Jedd

[slembcke]

No, there is no built in way to do it, and yes you will have to apply forces.

[gravity]

I've been thinking about doing the same thing,

I iterated over two loops

Ball is just a class that has a cpBody property - "ballBody." "bodies" is an array of all of the bodies on the screen. I call this in my main game loop "tick" :

Code: Select all

for (int i = 0; i < [bodies count]; i++)
	{
		Ball *bodyi = [bodies objectAtIndex:i];
		
		for (int j = i + 1; j < [bodies count]; j++) //notice that iteration goes from i+1, not 0
		{
			Ball *bodyj = [bodies objectAtIndex:j];
			
			cpVect displacement = cpvsub(bodyi.ballBody->p, bodyj.ballBody->p); //get distance between the two bodies
			cpVect normDisplacement = cpvnormalize(displacement); //normalize this
			float forceScale = -400000* bodyi.ballBody->m * bodyj.ballBody->m / (cpvlengthsq(displacement)); 
//and multiply it by the magnitude of the force
			cpVect force = cpvmult(normDisplacement, forceScale);
			
			bodyi.netForce = cpvadd(bodyi.netForce, force); 
			bodyj.netForce = cpvsub(bodyj.netForce, force);			
		}				
	}

and then later on apply the netForce to each body.

This seems to work, except, I'm getting an outrageous increase in speed over time. I do apply cpBodyResetAllForces before hand, and the netForces all add to 0, so I don't know where the crazy increase in energy is coming from.

Hope this helps, and if anybody has anything to say on it, it is appreciated.

[gravity]

lol.

My... solution... if you can call it that,

Code: Select all

	for (int i = 0; i < [bodies count]; i++)
	{
		Ball *bodyi = [bodies objectAtIndex:i];
		if (cpvlengthsq(bodyi.netForce) > 2000000) {
			bodyi.netForce = cpvmult(bodyi.netForce, 1/1.2);
			if (cpvlengthsq(bodyi.netForce) > 2500000) {
				bodyi.netForce = cpvmult(bodyi.netForce, 1/1.5);
				if (cpvlengthsq(bodyi.netForce) > 3000000) {
					bodyi.netForce = cpvmult(bodyi.netForce, 1/1.8);	
					if (cpvlengthsq(bodyi.netForce) > 3500000) {
						bodyi.netForce = cpvmult(bodyi.netForce, 1/2.5);
						if (cpvlengthsq(bodyi.netForce) > 4500000) {
							bodyi.netForce = cpvmult(bodyi.netForce, 1/2.9);
							if (cpvlengthsq(bodyi.netForce) > 9500000) {
								bodyi.netForce = cpvmult(bodyi.netForce, 0);
							}
						}
					}
				}
			}
		} 
		
		cpBodyApplyForce(bodyi.ballBody, bodyi.netForce, CGPointZero);
	}