Question Ball Bounce Physics Exit Trajectory

Hey all,

I'm pretty seasoned in Unity but I have yet to try and use the physics system as a core game mechanic & in doing so I might be driving myself crazy. In this case, the mechanic is simply a bouncing ball. I've stripped back everything to the simplest example.

In the gifs attached you'll see two balls that have the same rigidbody configuration. They are also simulated in the same scene, w/ the same floor physics material, etc. Everything is identical EXCEPT the launch trajectory. They are slightly different. Very, very, slightly different. I've also included the angular velocity magnitude & speed above the ball so as to better compare them, though I think it's likely unnecessary.

What you'll see is that each ball has a very different bounce trajectory. One "reflects" almost perfectly the incoming trajectory. The other ends up w/ a more vertical exit trajectory, which seems quite incorrect. The only thing I could think of that would actually result in a more vertical bounce is if the ball had backspin - which it does not.

Second to backspin, the only other difference I can see while debugging is that the sharper, "incorrect" bounce has more penetration. My guess is that more penetration results in a larger "de-penetration" force that is then added to the resulting velocity. Is that the case? If not, what is causing this sharp exit angle? If it is in fact because of more penetration, how can one expect a physical model to "feel" real if it has such unrealistic behavior in such a simple scenario?

P.S. I totally plan on just writing my own simulation for this, as the physics engine seems unlikely to produce consistent enough results for a model that could be easily hand-built. Also, given it's a core mechanic & quite simple it makes sense to write my own kinematic code to ensure maximum control. I'm simply digging into this as an academic endeavor - I want to understand my tools thoroughly.

"Good Bounce" - Exit trajectory near reflection of enter.


"Bad Bounce" - Exit trajectory much more vertical.

 

I agree with the penetration hypothesis.

Try reducing in your tests the fixed time step and see what happens.

You can also try a different collision detection
https://docs.unity3d.com/Manual/ContinuousCollisionDetection.html

 

Hey, a reply! Yayyyy .

Yeah, I can totally try reducing the timestep. That will obviously result in smaller distances traveled per physics update and thus reduce penetration. In the end, I'll just end up writing custom kinematics for this so it's more stable. Increasing the fixed timestep isn't really tractable given performance constraints.

Fwiw, I've tried all the collision detections schemes. This video was recorded w/ Discrete. Continuous speculative is worse & Continuous Dynamic results in the same behavior as Discrete.

My main confusion is... is this really how this works? It's really baffling to me. It seems "trivial" to have the bounce occur "accurately" - but I imagine that's because I'm not writing a generalized physics engine. I know it's just a ball & should behave a particular way.

In any case, I would have expected a continuous collision check to _stop_ the ball from penetration & reflect correctly....

Thanks again for the reply!

 

I believe these recordings are from a box collider. Fwiw, I don't think the collider would affect the amount of penetration. Should just be a function of physics time step & projectile speed. My confusion is simply that I wouldn't expect the amount of penetration to change how the collision is solved. In my mind, the penetration is re-wound, the collision point calculated, and the ball reflected - but that doesn't seem to be the case. I think it might be that the friction applied varies depending on the penetration as a frictionless physics material does not produce the same issues.

 

Can you maybe try with Unity Primitives?
I have tried myself with different angles and velocities and the bounces seem ok.