Why are simple sphere collisions with bounciness 1 not elastic?

The Setup

Using Unity 2018.4 I've created a scene of identical spheres, each of which has a SphereCollider and a Rigidbody:



Each sphere has a mass of 1, 0 friction, gravity disabled, and shares the same PhysicsMaterial with bounciness set to 1:



Using a trivial script I can set the initial velocity of some of the spheres:

Code (CSharp):

1. public class InitialVelocity : MonoBehaviour
2. {
3.     public Vector3 startVelocity;
4.  
5.     void Start()
6.     {
7.         GetComponent<Rigidbody>().velocity = startVelocity;
8.     }
9. }
10.  

And set the initial velocities of the spheres as follows:



The observed collisions all appear to be completely inelastic.

The Question

Why is this the case? Why are collisions between spheres with bounciness set to 1 not elastic? In fact in this case modifying the value of bounciness appears to have zero impact on the outcome.

Unity's documentation describes PhysicsMaterial.Bounciness as a value of 1 meaning zero loss of energy - textbook definition of an elastic collision - and yet that is not what is observed. Assuming standard physics textbook rules of not losing energy to heat, there is a clear loss of energy in the first set of spheres colliding and canceling out each other's momentum completely. For energy to be conserved in this scenario they would need to reflect off each other at velocities equal in magnitude and opposite in direction.

I am not asking for instructions on how to implement my own physics handling for elastic collisions between spheres. That is absolutely trivial here.

I ask this question to understand why the observed behavior in this specific scenario appears to be in direct violation of the documentation and the common sense interpretation of what it means for two objects with 100% "bouniness" to collide.

What is actually going on here? Am I misinterpreting how Unity defines bounciness?

 

That seems to work.


With some further testing I noticed that a newly created PhysicsScene appears to internally use the default bounceThreshold value, and not the one that was set before it was created. If I set Physics.bounceThreshold again after the PhysicsScene's creation it does appear to now use the new value.

So it appears PhysicsScene maintains its own internal bounceThreshold state, but after creation any changes to the static one will write to it as well.

Since it has its internal state, is there a way to modify that separately?

 

As related question, bounceThreshold is now a global setting. Could it be turned into a per-rigidbody (or per-material) setting, or the way it works now is related to the PhysX design? Like specifying a "Physics.defaultBounceThreshold" value that could be changed later per-object. I could make very good use of that setting in a more granular way.

 

Thanks for your help! I've submitted it as case 1181249