CODEWITHSUNDEEP
actionscript-3box2d
Phil
Phil

Reputation: 3043

Why does this produce a bounce in box2d?

I'm trying to get my box2d box to bounce when hitting another box2d box. The physical properties to both objects are set thus..

fixtureDef.density=1;
            fixtureDef.friction=0;
            fixtureDef.restitution=0.9;

And when I apply this line (which is meant to be the correct way of doing things) I get no bounce.

myElementsArray[0].myPhysics.myBody.ApplyImpulse(newVec, myElementsArray[0].myPhysics.myBody.GetWorldCenter());

But when I change the 2nd, part to some random vector, it bounces

myElementsArray[0].myPhysics.myBody.ApplyImpulse(newVec, new b2Vec2(20,20));

Which I don't get, what am I doing wrong?

Upvotes: 0

Views: 969

Answers (1)

Bennett Keller
Bennett Keller

Reputation: 1714

The "bounce" you are seeing is probably a bit deceptive. I'm not a physics guru, I only know enough be dangerous as my main focus is game programming. Anyway here goes, the first statement uses the center of mass to apply the force to. When you are calculating a "bounce" using restitution, in most physics libraries it would very simply grab the largest restitution between the two collision objects:

restitution = Max( bodyA.restitution, bodyB.restitution )

Then disregarding torque, the new differential final velocities with respect to the restitution is:

–restitution * (bodyA.initialVelocity - bodyB.initialVelocity)

This is also more standardly written as:

–e(V1i - V2i)

Why is this important? Because this is probably the reason you are not seeing a bounce, the two initial velocities (without torque) are zeroing out. Now the second call to ApplyImpulse() uses a coordinate that is not the center of mass of the body. Because of this, Box2D is going to attempt to apply an angular velocity to the body. With that in mind a new series of equations will be used. I'm fairly confident that is where the problem resides, but someone else might correct me or know more about it.

Upvotes: 1

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