Rigid Body Physics forces no longer restricted to specific layers/collections #87458

New Issue

Rich Sedman · 2021-04-13T10:16:21+02:00

Rich Sedman commented

2021-04-13 10:16:21 +02:00

System Information
Operating system: Windows-10-10.0.18362-SP0 64 Bits
Graphics card: Intel(R) Iris(R) Plus Graphics Intel 4.5.0 - Build 26.20.100.7641

Blender Version
Broken: version: 2.93.0 Alpha, branch: master, commit date: 2021-03-24 17:47, hash: 9ac7946dd0
Worked: (newest version of Blender that worked as expected)

Short description of error
Prior to Blender version 2.8, forces would only affect rigid body objects within the same layers. With later versions the scene is managed via Collections but despite forces being able to be placed in any number of collections, all forces seem to affect rigid bodies in all collections.

Note that saving a file with mulitple forces in different layers in Blender version 2.79b and loading into 2.93 Alpha does seem to retain the layer structure by creating an additional collection for the objects in different layers but the forces no longer work the same.

Exact steps for others to reproduce the error
Add Rigid Body Physics to the default cube and duplicate it to a second cube. Move the second cube into a different collection. Switch off Scene Gravity. Create two forces that would affect the two cubes in different ways (eg, place one above one cube and the other below the other cube) and move the forces to the respective collection relating to its associated cube. This should result in each force affecting only one of the cubes (since they are in different collections). However, running the simulation results in both cubes being affected by both forces - rather than just the force in its own collection.

**System Information** Operating system: Windows-10-10.0.18362-SP0 64 Bits Graphics card: Intel(R) Iris(R) Plus Graphics Intel 4.5.0 - Build 26.20.100.7641 **Blender Version** Broken: version: 2.93.0 Alpha, branch: master, commit date: 2021-03-24 17:47, hash: `9ac7946dd0` Worked: (newest version of Blender that worked as expected) **Short description of error** Prior to Blender version 2.8, forces would only affect rigid body objects within the same layers. With later versions the scene is managed via Collections but despite forces being able to be placed in any number of collections, all forces seem to affect rigid bodies in all collections. Note that saving a file with mulitple forces in different layers in Blender version 2.79b and loading into 2.93 Alpha does seem to retain the layer structure by creating an additional collection for the objects in different layers but the forces no longer work the same. **Exact steps for others to reproduce the error** Add Rigid Body Physics to the default cube and duplicate it to a second cube. Move the second cube into a different collection. Switch off Scene Gravity. Create two forces that would affect the two cubes in different ways (eg, place one above one cube and the other below the other cube) and move the forces to the respective collection relating to its associated cube. This should result in each force affecting only one of the cubes (since they are in different collections). However, running the simulation results in both cubes being affected by both forces - rather than just the force in its own collection.

Rich Sedman commented

2021-04-13 10:16:21 +02:00

Added subscriber: @BaldingWizard

Rich Sedman commented

2021-04-13 10:24:49 +02:00

Blender 2.79b physics force multi-layer test.blend

Loading the above file in Blender 2.79b produces the expected result (where one cube is accelerated upwards and the other down). Opening in 2.93Alpha produces unexpected results (with both cubes being affected by both forces - despite the file being loaded with the different forces and cubes being in different collections (so the loading of the layers works into separate collections but those collections aren't being interpreted in the same way as previously).

At 2.79b :

At 2.93Alpha (same file) :

[Blender 2.79b physics force multi-layer test.blend](https://archive.blender.org/developer/F9931529/Blender_2.79b_physics_force_multi-layer_test.blend) Loading the above file in Blender 2.79b produces the expected result (where one cube is accelerated upwards and the other down). Opening in 2.93Alpha produces unexpected results (with both cubes being affected by both forces - despite the file being loaded with the different forces and cubes being in different collections (so the loading of the layers works into separate collections but those collections aren't being interpreted in the same way as previously). At 2.79b : ![good physics.gif](https://archive.blender.org/developer/F9931536/good_physics.gif) At 2.93Alpha (same file) : ![bad physics.gif](https://archive.blender.org/developer/F9931538/bad_physics.gif)

Philipp Oeser commented

2021-04-13 10:59:11 +02:00

Added subscribers: @JacquesLucke, @ZedDB, @brecht, @lichtwerk

Philipp Oeser commented

2021-04-13 10:59:11 +02:00

Changed status from 'Needs Triage' to: 'Confirmed'

Philipp Oeser commented

2021-04-13 10:59:11 +02:00

I am a bit on shaky ground, but it indeed looks like this has changed to the worse here.

Documentation is not very clear here as well:
https://docs.blender.org/manual/en/dev/physics/rigid_body/world.html

You can create several Rigid Body World Collections and allocate the rigid body objects with the Collections panel.
Rigid body objects and constraints are only taken into account by the simulation if they are in the collection specified in the Collection field of the Rigid Body World panel in the Scene tab.

But it is unclear how you would actually have multiple collections in parallel.
And while the "old-style" layers for collisions was kept, this was not done for forces.

Afaict, you can still split this up in view layers / renderlayers like so:
Blender 2.93 physics force multi-layer test_viewlayers.blend
(but that comes with its own set of restrictions -- you then have to deal with comping these layers back together).

Summoning insights from (module) devs here on how the current design is supposed to work for rigid bodies and forces (but will confirm for now).
CC @JacquesLucke
CC @brecht
CC @ZedDB

I am a bit on shaky ground, but it indeed looks like this has changed to the worse here. Documentation is not very clear here as well: https://docs.blender.org/manual/en/dev/physics/rigid_body/world.html > You can create several Rigid Body World Collections and allocate the rigid body objects with the Collections panel. > Rigid body objects and constraints are only taken into account by the simulation if they are in the collection specified in the Collection field of the Rigid Body World panel in the Scene tab. But it is unclear how you would actually have multiple collections in parallel. And while the "old-style" layers for **collisions** was kept, this was not done for forces. Afaict, you can still split this up in `view layers` / `renderlayers` like so: [Blender 2.93 physics force multi-layer test_viewlayers.blend](https://archive.blender.org/developer/F9931561/Blender_2.93_physics_force_multi-layer_test_viewlayers.blend) (but that comes with its own set of restrictions -- you then have to deal with comping these layers back together). Summoning insights from (module) devs here on how the current design is supposed to work for rigid bodies and forces (but will confirm for now). CC @JacquesLucke CC @brecht CC @ZedDB

Sebastian Parborg commented

2021-04-13 17:08:02 +02:00

I think this is something that was lost when we removed the viewport layers/groups.

As Philipp pointed out, you can no longer separate the forces because of this.

However most of the rigid body code should probably be rewritten as there are many quirks and bugs that happens because of the current state of it.

I think this particular issue could be solved by copying the "collision collection" methodology from the cloth sim.
IE you can specify a collection that has effectors that should exert forces on the rigid body in question.
If no collection is specified, all effectors in the scene will exert forces on it.

However I'm currently working on fixing and adding feature for the animation system.
So I don't know when I will have time allocated to work on this.

Therefore, I will mark this as a known issue.

If someone wants to work on this, they can ping me and I'll try to provide guidance.

I think this is something that was lost when we removed the viewport layers/groups. As Philipp pointed out, you can no longer separate the forces because of this. However most of the rigid body code should probably be rewritten as there are many quirks and bugs that happens because of the current state of it. I think this particular issue could be solved by copying the "collision collection" methodology from the cloth sim. IE you can specify a collection that has effectors that should exert forces on the rigid body in question. If no collection is specified, all effectors in the scene will exert forces on it. However I'm currently working on fixing and adding feature for the animation system. So I don't know when I will have time allocated to work on this. Therefore, I will mark this as a known issue. If someone wants to work on this, they can ping me and I'll try to provide guidance.

James Tomkinson commented

2021-04-13 17:38:41 +02:00

Added subscriber: @jamestomk

Christian Brinkmann commented

2021-11-15 00:09:09 +01:00

Added subscriber: @p2or

Philipp Oeser removed the

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Rigid Body Physics forces no longer restricted to specific layers/collections #87458