Sign in to follow this  

3D Sphere Collisions

Recommended Posts

I've incorporated some pretty basic 3D sphere collisions and response. It works pretty well in about 95% of all cases but in certain situations where the spheres start in positions that are already colliding by a substantial amount, the spheres start to orbit around each other in a pretty interesting manner, though definitely not what I'm looking for. The code is posted below, the collision response is borrowed from gamedev, and I'm wondering if anyone knows what might be causing them to orbit rather than bounce off each other like they normally do. Thanks in advance.

        CVector3 difference = position - boulder->position;
	float d_diff = Magnitude(difference) - (boulder->radius + radius);

	if(d_diff <= 0)

		float f_diff1 = weight / (boulder->weight + weight);
		float f_diff2 = boulder->weight / (boulder->weight + weight);
		CVector3 pb1,pb2,xaxis,U1x,U1y,U2x,U2y,V1x,V1y,V2x,V2y;
		double a,b;
		xaxis=Normalize(pb2-pb1);							// Find X-Axis
		a=Dot(xaxis,velocity);					// Find Projection
		U1x=xaxis*a;								// Find Projected Vectors
		xaxis=Normalize(pb1-pb2);							// Do The Same As Above
		b=Dot(xaxis,boulder->velocity);					// To Find Projection
		U2x=xaxis*b;								// Vectors For The Other Ball
		V1x=(U1x*f_diff1+U2x*f_diff2-(U1x*f_diff1-U2x*f_diff2))*elasticity;						// Now Find New Velocities
		velocity=V1x+V1y;						// Set New Velocity Vectors
		boulder->velocity=V2x+V2y;						// To The Colliding Balls

		position = position + velocity*(time_diff);
		boulder->position = boulder->position + boulder->velocity*(time_diff);	


Share this post

Link to post
Share on other sites
You have to do a swept test, it gets rid of the problem if they are moving too fast that they jump through each other.

its basically the distance formula combined with the velocities*time minue radius1+radius2

and u use the quadratic equation to make time = the rest

Vector p = objects[i]->position - objects[j]->position;
Vector v = objects[i]->velocity - objects[j]->velocity;
double r = objects[i]->radius + objects[j]->radius;

//quadratic equation
double a = v.x*v.x + v.y*v.y + v.z*v.z;
double b = 2*(p.x*v.x + p.y*v.y + p.z*v.z);
double c = p.x*p.x + p.y*p.y + p.z*p.z - r*r;
double det = b*b - 4*a*c; //determinant

if(det >= 0) {
double colTime = (-b - sqrt(det))/(2*a);

//using -deltaTime, incase the spheres are
//already colliding, it moves them back to
//the place where they would of collided
if(colTime >= -deltaTime && colTime < moveTime) {
//move the objects forward by this time
//then apply your collision response


Share this post

Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now

Sign in to follow this