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Compund_Collision_chain demo not working properly
Confirmed, NormalPublic

Description

System Information
Operating system: Linux-5.4.0-65-generic-x86_64-with-debian-bullseye-sid 64 Bits
Graphics card: GeForce GTX 1070/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 460.32.03

Blender Version
Broken: version: 2.92.0 Release Candidate, branch: master, commit date: 2021-02-22 12:21, hash: rBe497c1b93ca5
Worked: (newest version of Blender that worked as expected)

Short description of error
The demo file Compund_Collision_chain that can be downloaded from https://www.blender.org/download/demo-files/ it doesn't seem to be working properly

Exact steps for others to reproduce the error
1- Download the file compound_collision_chain.blend from https://www.blender.org/download/demo-files/

2-in the properties tab "scene properties" delete the bake for the rigid bodies

3- Probably after deleting the bake the baked keyframes are still in ram if you did playback before deleting the cache, add a keyframe to the position of the first chain link just to force blender to dump the cache.

4- This should happend if you bake again or try to playback the animation

Event Timeline

Philipp Oeser (lichtwerk) changed the task status from Needs Triage to Confirmed.Feb 24 2021, 10:18 AM

Can confirm.

@Sebastian Parborg (zeddb) : does this ring a bell?

Seems like something is wrong with the hosted test file!

If you use the original file from the patch, it works: https://developer.blender.org/D5797

I've have seen this issue before and it seems like the bullet physics world has somehow gotten into an invalid state.
So no matter what you do, things will explode and get "ghost" forces applied to them.

Previously I had to delete the rigidbody physics world and start over in the file to fix this.
I don't know why or how this happens, perhaps it has to do with compression as the original example file is ~6mb while the newer broken one is ~800kb...

@Nahuel Belich (belich) can you see if you can reproduce the issue by starting with the original working file and then try to make it break like in the new example file?
I'm guessing it is related to saving and loading the file.

@Nahuel Belich (belich) Great!
Note the I have never myself been able to reproduce broken files it is only from bug reports that I have gotten them.
So if you could figure out how to break it that would be super!

@Sebastian Parborg (zeddb)
This is becoming a bit weirder than my artist brain can understand, first the tests.


Blend file "Comptest Chain Original.blend" From https://developer.blender.org/D5797

2.91.2 : Works fine
2.92 candidate : Works fine
2.93 alpha : Works fine


Blend File "compound_collision_chain.blend" from https://www.blender.org/download/demo-files/

2.91.2 : fails
2.92 Candidate : fails
2.93 alpha : fails

This is the weirdest part, i though that some property could be different between objets in the files and the properties looks all the same, then i though may be the compress option for the blend file, didn't change anything, then i try this.

In the old demo the one that weights 6mb
1 - select a few links of the chain
2- move it a bit to one side
3- Ctrl+c to copy
4- Ctrl+v to paste the object in a new empty blend file
did the same with the file hosted in blender.org the one that weights 769 Kb, move the chains same way but to the other side, shoud look the same under a new empty blender but. .



as you can see the chains looks very different, the one on the right from the hosted blender.org file, twists into itself in a really weird way.
i check the objetcs one by one and the rigid bodies properties are the same at least in the interface that i can see, should look the same but it does not.

I made a test from scratch, I made new objects in a new file and copy all the parameters by hand for the rigid body from+ the chain in the hosted blender.org file, the ones that looks bad in the video above, and it works as a charm in this new file.


i wasn't able to crash the original file, save with compression with out compression, baked, no baked, not sure whats happening but as it looks what its breaking things can be copy with the objects if you copy objects from blend to blend.

David's original scene Comptest Chain.blend (6mb) used three separate cubes each with a convex hull. This one works.


Then Dalai did some "cleanup" compound_collision_chain.blend (777kb) and merged those cubes, creating a concave shape that still has convex hull. This is what you download off of Blender. Doesn't work.

David then fixes this compound_collision_chain.blend (253kb) by separating the bottom cube and removing the rigid bodies from the wall.

David explains this in the last comment of that thread.

As for the chain behaving differently, I also couldn't find anything different. Same numbers, same parenting, same orientation, same structure, but obviously something happened. What you can see is that only two links of the top chain seem to be different.

@Fernando Alcala (TriFractal) what thread are you talking about here. I'm a bit lost at it seems like you are quoting a forum thread but not linking to it.