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Dependency Cycle Detected when copying objects with cloth and collision modifier
Confirmed, NormalPublicKNOWN ISSUE


System Information
Operating system: Windows 10
Graphics card: Geforce GTX 1080 Ti

Blender Version
Broken: 2.80

Short Description
If you copy objects that have a collision and cloth modifier attached to them (or copy the modifiers to another object), a dependency cycle is detected. This is really frustrating once you work with 10+ objects because blender re-detects the circular dependencies on every redo/undo and project load, making you wait for a good 30 seconds to a minute depending on the complexity of the scene.

Exact steps for others to reproduce the error

  1. attach collision and cloth modifier to cube in startup project.
  2. Copy the cube.
  3. check console output.

The output should be something like this:

Dependency cycle detected:
  OBCube.001/Geometry Component/GEOMETRY_EVAL() depends on
  OBCube/Geometry Component/GEOMETRY_EVAL() via 'Cloth Collision'
  OBCube.001/Geometry Component/GEOMETRY_EVAL() via 'Cloth Collision'
Dependency cycle detected:
  OBCube.001/PointCache Component/POINT_CACHE_RESET() depends on
  OBCube/Geometry Component/GEOMETRY_EVAL() via 'Point Cache'
  OBCube.001/Geometry Component/GEOMETRY_EVAL() via 'Cloth Collision'
  OBCube.001/PointCache Component/POINT_CACHE_RESET() via 'Point Cache -> Geometry'
Detected 2 dependency cycles

Alternatively you can simply open the attached blend file and check the console output. For a simple test like this, it wont noticeably slow down the project but once you copy an object a couple of times the dependency circles appear to exponentially grow and slowing things down to a halt.

Event Timeline

Jacques Lucke (JacquesLucke) lowered the priority of this task from 90 to 30.Sep 19 2019, 5:28 PM

Did this work in older versions of Blender? I'm not entirely sure which kinds of interactions the current cloth system supports.

Did this work in older versions of Blender? I'm not entirely sure which kinds of interactions the current cloth system supports.

I am a new user to blender, so I would not know. I'll try to test it.

For complex scenes with a lot of identical cloth objects this bug really bites you in the butt because you can neither copy the objects directly nor copy the physics modifiers from one object to another without running into this issue. It will essentially make the whole blender file unusable because the dependency cycles will exponentially grow and it will re-detect them on every action, undo, redo, project load.

Bastien Montagne (mont29) raised the priority of this task from 30 to 80.Oct 1 2019, 9:21 PM

This could very well be a limitation of cloth/physics.

Not really familiar with details of those areas, but same exact issue was happening for a long long time.

Richard Antalik (ISS) changed the task status from Needs Information from Developers to Confirmed.Jan 10 2020, 9:45 PM
Bastien Montagne (mont29) changed the subtype of this task from "Report" to "Known Issue".Jan 15 2020, 4:54 PM

Note that copying is irrelevant here, you can just as well create two meshes, and enable those collision and cloth for both, and you'll get same dependency cycles errors in console. Would consider that as a known issue for now…

Hi all, just a quick comment here. I am using Blender 2.82a and posted an issue report on StackExchange.

I was then linked to this bug report. (Apologies for the semi-duplicate message on StackExchange, as I did not see the bug report in my investigation.) My message is mostly to confirm that this is still happening with Blender 2.82a, but it *might* also have a harmless effect in terms of physics. I can sometimes get cloth and collisions working reasonably well in my animations.

Anyway, hopefully my report is another useful data point. Thanks everyone for the great software! I'm still blown away by the quality.

Hi Daniel,

Just to clarify, the problem does not break the physics simulation (that looks fine in all my tests) but rather causes everything (mainly the undo/reco action stack) to slow down to a crawl and make things pretty much unusable with a certain number of objects in the scene/simulation.