Phys2D: Catching large velocity changes

Game Mechanics
#1

I want to catch when there is a large change in a Body’s velocity, basically to deal with “violent” collisions and the like. I tried to check to see if Math.abs(b.getVelocity().length()-b.getLastVelocity().length()) >= 100, (where b is a Body) but nothing happened. So, I printed out the changes every timestep, and even when I put gravity up to 50 and drop a Body from very high up, when it collides apparently the velocity difference is, at the most, 0.0072310437. I know that this is very, very, wrong. Not to mention the fact that while falling the Body still reported a velocity change of 0, even though it was being accelerated by gravity.

Any hints here? I also tried checking as collision happens, but that didn’t work because by the time the Arbiters were able to find a collision it appeared that the falling object had already had its opposite force applied and whatnot.

#2

I think your calculation is wrong, since before and after collision, the velocity should be almost equal in length but not the same direction.

Try something like :


  Vector2f v = new Vector2f(b.getVelocity());
  v.sub(b.getLastVelocity());

  if (v.length() >= 100) { ... }
#3

When falling, they should be equal in direction and not at all equal in length, however, I do see your point in a potential error (perfect reflection with no energy loss). The fact is, something is working wrong for me. I’ve done a lot of testing and lastVelocity and velocity are almost always exactly the same, and when they deviate it’s incredibly tiny.

I ended up getting this to work, but only by creating my own lastVelocty variable that I store in my wrapper class for Body’s. That way, when the change is 250 m/s, the change that gets reported is 250 m/s. It’s just ineloquent and I feel like there’s got to be something in Phys2D that can do this for me.

#4

Well, when falling with a 1.0 restitution, the velocity has the same direction, the same length but another “orientation” (sorry i don’t know the right words).

Neverless, it remind me a problem that i had with getLasPosition(). If it is the same, it is use in the collission calculation and should not be use outside (or something like that). You should backup the velocity before the step().

#5

Yeah, that’s what I’m doing right now. It’s not beautiful, but works.

#6

Well, I had this same problem in my current game project (SECTOR91.com’s Project Delta), and I took a completely different approach to this. I didn’t even bother to measure the velocity change; I just factored in the mass of each object and the velocity that each was traveling at when they collide. The objects (the class is called DDestructableBody, a Body with an HP value) have a function called applyForce() which takes the force of the collision and compares it with a damage threshold, and if the force is over the threshold the object takes damage. In order to do this, my World wrapper class has a collision listener function that looks like this:


public void collisionOccured(CollisionEvent e)
	{
		float mass1 = e.getBodyA().getMass();
		if (mass1 == Float.POSITIVE_INFINITY || e.getBodyA().isResting())
			mass1 = 1;
		float mass2 = e.getBodyB().getMass();
		if (mass2 == Float.POSITIVE_INFINITY || e.getBodyB().isResting())
			mass2 = 1;
		float force = ((Math.abs(e.getBodyA().getVelocity().getX() * e.getNormal().getX()) + Math.abs(e.getBodyA().getVelocity().getY() * e.getNormal().getY())) * e.getBodyA().getMass()) + ((Math.abs(e.getBodyB().getVelocity().getX() * e.getNormal().getX()) + Math.abs(e.getBodyB().getVelocity().getY() * e.getNormal().getY())) * e.getBodyB().getMass());
		if (e.getBodyA() instanceof DDestructableBody)
		{
			DDestructableBody d = (DDestructableBody)e.getBodyA();
			d.applyForce(force);
		}
		if (e.getBodyB() instanceof DDestructableBody)
		{
			DDestructableBody d = (DDestructableBody)e.getBodyB();
			d.applyForce(force);
		}
	}

There is one problem with this currently: it doesn’t take into account the direction of the forces, so if two bodies moving in the same direction collide they will take way too much damage. I just haven’t taken the time to fix this yet.