Real-Time Physics Simulation Forum

Re: Unstable behavior on shapes like cylinder or box

2022-02-23T15:05:58+00:00

It seems to use rolling friction you need to also enable anisotropic rolling friction. Doing so makes the rolling friction work without objects standing on the edges that much. There is still some angles at which they stand still but I'll go through the bullet examples and collect the parameters used. Maybe I see something that changed. I'm also used to the old code and just updated from 2.87 to 2.89 . I guess I have to adjust some more values to get it working again. As good as in the examples would be enough for me. Right now a tower of cans for ball throwing won't stack beyond the bottom layer so there is something still off.

Statistics: Posted by Dragonlord — Wed Feb 23, 2022 3:05 pm

Re: Unstable behavior on shapes like cylinder or box

2022-02-23T14:26:52+00:00

Yes interesting. I downloaded your source code this morning and was looking around. I checked the commit logs and saw that you may have made some progress on the problem. I then diffed with history to browse the relevant changes.

To tell the truth, I don't have any experience with rolling/spinning friction. It looks like they were added/enabled back in late 2019 and all of my personal Bullet experience is prior to that year (maybe I should stop trying to answer questions on this forum). It is possible you've encountered a new/real bug in the Bullet simulation.

Statistics: Posted by drleviathan — Wed Feb 23, 2022 2:26 pm

Re: Unstable behavior on shapes like cylinder or box

2022-02-23T00:56:56+00:00

I've done some more experimenting and disabled this and that rigid body option until I got a change. It looks like the rolling friction cause the problem. I used the value from the RollingFrictionDemo which is 0.1f . Using this value the cylinder does not roll but can even stand on its edge at 80 degrees. If I set the rolling friction to 0 (which is the default) then the edge standing problem is gone but the cylinder starts rolling away accelerating while doing so.

How can I thus prevent cylinders from rolling (and accelerating) on the floor without standing ridiculously on their edge? The demo values do not seem to work.

Statistics: Posted by Dragonlord — Wed Feb 23, 2022 12:56 am

Re: Unstable behavior on shapes like cylinder or box

2022-02-22T21:46:15+00:00

Ok that debpMotionState looks fine: I don't see any misbehaving logic there.

Any ideas where I could put a lever to debug this problem? I tried examining the contact constraint generated but I did not get the impression something is wrong. That said I'm not much into physics coding so I don't know what kind of values would look wrong.

Statistics: Posted by Dragonlord — Tue Feb 22, 2022 9:46 pm

Re: Unstable behavior on shapes like cylinder or box

2022-02-22T17:50:11+00:00

Ok that debpMotionState looks fine: I don't see any misbehaving logic there.

Statistics: Posted by drleviathan — Tue Feb 22, 2022 5:50 pm

Re: Unstable behavior on shapes like cylinder or box

2022-02-22T17:14:46+00:00

The beer can... when it lands on the ground does it get deactivated? or does it remain active?

Are you using a custom MotionState in your game? If so, what does its getWorldTransform() and setWorldTransform() implementations look like?

Yes, a motion state is used. The code is here: https://github.com/LordOfDragons/dragen ... nState.cpp

Statistics: Posted by Dragonlord — Tue Feb 22, 2022 5:14 pm

Re: Unstable behavior on shapes like cylinder or box

2022-02-22T16:55:00+00:00

The beer can... when it lands on the ground does it get deactivated? or does it remain active?

Are you using a custom MotionState in your game? If so, what does its getWorldTransform() and setWorldTransform() implementations look like?

Statistics: Posted by drleviathan — Tue Feb 22, 2022 4:55 pm

Re: Unstable behavior on shapes like cylinder or box

2022-02-22T12:03:41+00:00

The box and cylinder shapes are not unstable. I've seen them work fine.

I don't know what is wrong with your beer can. It behaves as if... the moment it hits the ground its world-frame inverse inertia tensor gets zero elements in the two axes perpendicular to the UP direction. That is: it is constrained to only rotate about world-frame UP axis. I can't imagine what would do that except for rogue custom code that is somehow (a) messing with the object's inertia tensor, (b) applying a hinge constraint or (c) some Action that is incorrectly slamming the object's angular velocity every frame.

Friction can be set per-object via btCollisionObject::setFriction(), but I assume you've been trying that. I've seen situations where some bodies never settle and instead just rattle around. As I recall they was caused by: (1) incorrect inertia tensor for tiny objects, (2) high restitution, zero damping, and too-small sleeping thresholds, and (3) a bug where custom Action was incorrectly re-activating the object every frame.

I added a printf to check the inertia tensor. I have the impression it is okay. At last I see no zero's:
updateInertiaTensor: 0x555abd8c9a80 [83.7424,9.44874,-22.1889][9.44874,92.087,-34.0537][-22.1889,-34.0537,157.556]

This method is called every call to stepSimulation(). Is this correct?

I also put a printf on setAngularVelocity in btRigidBody. There is no call to this method so none of my code messes with the angular velocity without my knowledge.

I'm also adding no constraint.

Any other ideas what could be wrong? How can I approach debugging this issue?

EDIT: I tried getting values from a single run of setupContactConstraint of the situation above where the can is nearly flat on the ground spinning. Maybe you see some value which is glaringly off or broken? https://pastebin.com/7VaYd9U0

EDIT: EDIT: I've also logged all updateInertiaTensor() calls of the object. At no time zero values are present in the tensor.

Statistics: Posted by Dragonlord — Tue Feb 22, 2022 12:03 pm

Re: Unstable behavior on shapes like cylinder or box

2022-02-22T01:42:30+00:00

The box and cylinder shapes are not unstable. I've seen them work fine.

I don't know what is wrong with your beer can. It behaves as if... the moment it hits the ground its world-frame inverse inertia tensor gets zero elements in the two axes perpendicular to the UP direction. That is: it is constrained to only rotate about world-frame UP axis. I can't imagine what would do that except for rogue custom code that is somehow (a) messing with the object's inertia tensor, (b) applying a hinge constraint or (c) some Action that is incorrectly slamming the object's angular velocity every frame.

Friction can be set per-object via btCollisionObject::setFriction(), but I assume you've been trying that. I've seen situations where some bodies never settle and instead just rattle around. As I recall they was caused by: (1) incorrect inertia tensor for tiny objects, (2) high restitution, zero damping, and too-small sleeping thresholds, and (3) a bug where custom Action was incorrectly re-activating the object every frame.

Statistics: Posted by drleviathan — Tue Feb 22, 2022 1:42 am

Re: Unstable behavior on shapes like cylinder or box

2022-02-21T16:07:12+00:00

I've added a small video showing the problem.

Statistics: Posted by Dragonlord — Mon Feb 21, 2022 4:07 pm