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Bug on Rigid Body Simulation when an object is parented to another
Confirmed, NormalPublicKNOWN ISSUE


System Information
Operating system: Windows 10
Graphics card: GTX 1060 6G

Blender Version

Short description of error
Bug on Rigid Body Simulation when an object is parented to another

Exact steps for others to reproduce the error
Please check the video......

More, I have some questions on Rigid Body. I want to ask something, coding and algorithm, to the developers who work on it...
How can I reach them?


Event Timeline

Can confirm in Blender 2.8 and today daily build 2.81..
I think, the expected behavior should be child object will be effected by both Parent (transform) and rigid body.
In this case as parent object is still on it's place so child object can freely act as rigid body

Philipp Oeser (lichtwerk) lowered the priority of this task from 90 to 80.Aug 23 2019, 10:59 AM

In 2.80, I can see object jumping around, but for me, 2.81/master is stable (in that it ignores the sim completely)

Think this is expected behavior since there is a dependency circle:

Dependency cycle detected:
  SCScene/Transform Component/RIGIDBODY_SIM() depends on
  OBSphere/Transform Component/TRANSFORM_SIMULATION_INIT() via 'Object Transform -> Rigidbody Sim Eval'
  OBSphere/Transform Component/TRANSFORM_EVAL() via 'Transform Eval -> Simulation Init'
  OBSphere/Transform Component/TRANSFORM_PARENT() via 'Eval'
  OBSphere/Transform Component/TRANSFORM_LOCAL() via 'ObLocal -> ObParent'
  OBSphere/Transform Component/TRANSFORM_INIT() via 'Transform Init'
  OBCube/Transform Component/TRANSFORM_FINAL() via 'Parent'
  OBCube/Transform Component/RIGIDBODY_TRANSFORM_COPY() via 'Rigidbody Sync -> Transform Final'
  SCScene/Transform Component/RIGIDBODY_SIM() via 'Rigidbody Sim Eval -> RBO Sync'

2.79 magically behaved "better" in that it seemingly completely ignored the parent transform.

I think, the expected behavior should be child object will be effected by both Parent (transform) and rigid body.

Dont think this is possible with the current system (unless we somewhat translate parent transforms into bullet forces?)

CC @Sergey Sharybin (sergey), @Sergej Reich (sergof)

Afraid we have to close this?

Lots of issues with parenting and rigid bodies in 2.8x it seems.
Encountered a similarly unpredictable effect with an empty parented to a rigid body object that uses the empty for its Simple Deform modifier.

Result is that the Simple Deform modifier acts as if the empty is moving (even though it's transforming around the object origin and thus with the mesh).
Haven't found a way to fix this other than just applying the modifier.

Yes, this has problems with the dependency graph [it is producing dependency cycles in 2.8, which is correct, whereas 2.79 somewhat made its way through in a hackish way...]
Sooo, dependency cycles are not treated as bugs [unless they are plain wrong -- which doesnt seem to be the case here]
Note that T58044: Issue with rigid body and modifiers dependent on transform is related.
Still need to think a bit about passive rigid bodies though [unsure whether a passive rigid bodies transform really depends on Scenes Rigidbody Sim?]...

Jacques Lucke (JacquesLucke) changed the task status from Needs Developer to Reproduce to Confirmed.Feb 12 2020, 3:25 PM
Jacques Lucke (JacquesLucke) changed the subtype of this task from "Report" to "Known Issue".

I set this as known issue, because it fixing it might require deeper changes to how Rigid Bodies are implemented in Blender and it is unlikely, that someone is fixing this in the next couple of month.