Rigid Body and Cloth simulations are indeterministic and borderline unusable in production #83594

New Issue

Tobiasz Karoń · 2020-12-09T15:31:16+01:00

Tobiasz Karoń commented

2020-12-09 15:31:16 +01:00

System Information
Operating system: Manjaro Linux
Graphics card: Radeon RX 5500/5500M / Pro 5500M

Blender Version
Broken: official stable 2.91.0

Short description of error

The rigid body simulation produces unrpredictible results that unexpectedly change without user input.

//This is my second time writing this, becuse the website suddenly imploded and I've lost all my progress.
Now I'm writing this in a text editor and I'll just paste it to save myself the trouble.//

Blender's rigid body simulations are terribly unstable. They produce sometiems wildly different results for no apparent reason.

I've spent days fighting an uphill battle trying to get a tall rigid body to fall down on a flat floor the way I want.
When I finally got it and been working on visual aspects of the scene, not touchign anything physics related, suddently the rigid body was falling in a different way, rendering my previous work wasted.

I also have a cloth object attached to the rigid body with a Pin vertex group - getting this to reliably collide with the rigid body, and not get crazy was hell.
But I did it. I've tweaked every setting dozens of times, animated the simulation properies like damping and friction to calm the cloth down after the initial impact wiht the floor, I've even animated the rigid body mesh with shapekeys to prevent the cloth form getting stuck on it.

I've searched for tips and done all I could to get the rigid body to be stable:

All objects scale is 1
I've made a simplified mesh for the rigid body and parented the visual mesh to it
I've made sure the collision mesh has no overlapping geometry
I've tweaked the quality and substeps settings, and spend days masaging the settings

I've finally got it to look like I wanted.
Nice. let's move on to the visual aspects of it.
I've saved a coouple new version of the project file, but because the pysics cache needs to be redone every time, I'm baking the physics agai nand again.

But now it all suddenly produces different results, even though I haven't touched the simultion aprameters. I've tried getting it to work again but now Blender crashes when I try to bake the simulation (I've reported that separately).

It's as if Blender has some secret random seed for physics and it changes that when it feels like it just to mess with me.

This is a simple slow-mo shot with two rigid bodies and a cloth sim. I can hardly imagine a simpler scene to simulate.

I should be done with this shot in a day tops. But due to the simulations going crazy all the time, I'm wasting yet another day on this.
Trying to get the rigid body and the cloth to behave feels like playing a ball maze, only there's 10 balls and there's an earthquake going on while I play.

I'll probably have to remake that entire shot from scratch and hope it's gonna somehow work this time.

I think it's great that Blender gets new exciting features in, but I think the developers should focus more on getting the existing features to be reliable.
IMO these kinds of problems are really preventing Blender from wider adoption in the industry.

Also: I've had problems with even rendering the animation propelry, when the physics seemed fine. In the preview the baked physics looked as expected, but in a rendered animation some frames would glitch back to a previous cached frame, resulting in a a lot of frames that are completely unusable. It initially happened less with Viewport > Render Animation, but at some point even that got me the glitched frames. Not to mention that way or rendering is greatly limiting my options.

I can't publish the specific project I'm working on, but I've made another test project, so you can see what kinds of issues I'm struggling with:

Rigid Body instability 03.blend

Rigid Body instability 02.blend

Rigid Body instability 01.blend

03.webm

02.webm

01.webm

I've previously written detailed explanation in what is wrong with these simulation, but I'm gonna give you a TL;DR version now:

The cloth collides with the rigid body only when it feels like it.
The rigid body is glitching out, colliding with non-existent objects and semingly chaning it's mass and center of mass.
Maybe after another 50 iterations I'd be able to get this to behave like I wanted for a while, but what's the point, if it's gonna break again next time I try to render it out?

I love Blender, I've been using it for decades now, and I think you guys are doing an amazing job with it, but right now Blender is giving me hell.

**System Information** Operating system: Manjaro Linux Graphics card: Radeon RX 5500/5500M / Pro 5500M **Blender Version** Broken: official stable 2.91.0 **Short description of error** The rigid body simulation produces unrpredictible results that unexpectedly change without user input. //This is my second time writing this, becuse the website suddenly imploded and I've lost all my progress. Now I'm writing this in a text editor and I'll just paste it to save myself the trouble.// Blender's rigid body simulations are terribly unstable. They produce sometiems wildly different results for no apparent reason. I've spent days fighting an uphill battle trying to get a tall rigid body to fall down on a flat floor the way I want. When I finally got it and been working on visual aspects of the scene, not touchign anything physics related, suddently the rigid body was falling in a different way, rendering my previous work wasted. I also have a cloth object attached to the rigid body with a Pin vertex group - getting this to reliably collide with the rigid body, and not get crazy was hell. But I did it. I've tweaked every setting dozens of times, animated the simulation properies like damping and friction to calm the cloth down after the initial impact wiht the floor, I've even animated the rigid body mesh with shapekeys to prevent the cloth form getting stuck on it. I've searched for tips and done all I could to get the rigid body to be stable: - All objects scale is 1 - I've made a simplified mesh for the rigid body and parented the visual mesh to it - I've made sure the collision mesh has no overlapping geometry - I've tweaked the quality and substeps settings, and spend days masaging the settings I've finally got it to look like I wanted. Nice. let's move on to the visual aspects of it. I've saved a coouple new version of the project file, but because the pysics cache needs to be redone every time, I'm baking the physics agai nand again. But now it all suddenly produces different results, even though I haven't touched the simultion aprameters. I've tried getting it to work again but now Blender crashes when I try to bake the simulation (I've reported that separately). It's as if Blender has some secret random seed for physics and it changes that when it feels like it just to mess with me. This is a simple slow-mo shot with two rigid bodies and a cloth sim. I can hardly imagine a simpler scene to simulate. I should be done with this shot in a day tops. But due to the simulations going crazy all the time, I'm wasting yet another day on this. Trying to get the rigid body and the cloth to behave feels like playing a ball maze, only there's 10 balls and there's an earthquake going on while I play. I'll probably have to remake that entire shot from scratch and hope it's gonna somehow work this time. I think it's great that Blender gets new exciting features in, but I think the developers should focus more on getting the existing features to be reliable. IMO these kinds of problems are really preventing Blender from wider adoption in the industry. Also: I've had problems with even rendering the animation propelry, when the physics seemed fine. In the preview the baked physics looked as expected, but in a rendered animation some frames would glitch back to a previous cached frame, resulting in a a lot of frames that are completely unusable. It initially happened less with Viewport > Render Animation, but at some point even that got me the glitched frames. Not to mention that way or rendering is greatly limiting my options. I can't publish the specific project I'm working on, but I've made another test project, so you can see what kinds of issues I'm struggling with: [Rigid Body instability 03.blend](https://archive.blender.org/developer/F9497201/Rigid_Body_instability_03.blend) [Rigid Body instability 02.blend](https://archive.blender.org/developer/F9497200/Rigid_Body_instability_02.blend) [Rigid Body instability 01.blend](https://archive.blender.org/developer/F9497199/Rigid_Body_instability_01.blend) [03.webm](https://archive.blender.org/developer/F9497204/03.webm) [02.webm](https://archive.blender.org/developer/F9497203/02.webm) [01.webm](https://archive.blender.org/developer/F9497202/01.webm) I've previously written detailed explanation in what is wrong with these simulation, but I'm gonna give you a TL;DR version now: The cloth collides with the rigid body only when it feels like it. The rigid body is glitching out, colliding with non-existent objects and semingly chaning it's mass and center of mass. Maybe after another 50 iterations I'd be able to get this to behave like I wanted for a while, but what's the point, if it's gonna break again next time I try to render it out? I love Blender, I've been using it for decades now, and I think you guys are doing an amazing job with it, but right now Blender is giving me hell.

Tobiasz Karoń commented

2020-12-09 15:31:16 +01:00

Added subscriber: @TobiaszunfaKaron

Jan-Willem van Dronkelaar commented

2020-12-10 01:36:46 +01:00

Added subscriber: @jwvdronkelaar

Jan-Willem van Dronkelaar commented

2020-12-10 01:36:46 +01:00

I know you did not post this report to get work arounds but just to save you some pain in the future. Once you are happy with your physics bake them out! It can be to keyframes for rigid bodies and a cache for cloth sim/soft bodies. Blenders own docs recommend it https://docs.blender.org/manual/en/latest/physics/baking.html

It is generally recommended to bake your physics simulations before rendering. Aside from no longer needing to go through the time-consuming process of simulating again, baking can help prevent potential glitches and ensure that the outcome of the simulation remains exactly the same every time.

You can even save the cache to disk and rename it to avoid accidentally overriding it even when you press bake again.

Of course this does not invalidate your report, its just meant as a helpful tip.

I know you did not post this report to get work arounds but just to save you some pain in the future. Once you are happy with your physics bake them out! It can be to keyframes for rigid bodies and a cache for cloth sim/soft bodies. Blenders own docs recommend it https://docs.blender.org/manual/en/latest/physics/baking.html > It is generally recommended to bake your physics simulations before rendering. Aside from no longer needing to go through the time-consuming process of simulating again, baking can help prevent potential glitches and ensure that the outcome of the simulation remains exactly the same every time. You can even save the cache to disk and rename it to avoid accidentally overriding it even when you press bake again. Of course this does not invalidate your report, its just meant as a helpful tip.

Richard Antalik commented

2020-12-11 22:17:31 +01:00

Added subscriber: @iss

Richard Antalik commented

2020-12-11 22:17:31 +01:00

Changed status from 'Needs Triage' to: 'Needs User Info'

Richard Antalik commented

2020-12-11 22:17:31 +01:00

Blender's rigid body simulations are terribly unstable. They produce sometiems wildly different results for no apparent reason.

Can you provide steps to reproduce this? I have been playing with cloth collision for quite some time, I can reproduce issues there, but not weirdness such as in your file.

If you can provide steps, I can understand your frustration, but it does really help if you make them as simple and clear as possible. See #83679 for example

>Blender's rigid body simulations are terribly unstable. They produce sometiems wildly different results for no apparent reason. Can you provide steps to reproduce this? I have been playing with cloth collision for quite some time, I can reproduce issues there, but not weirdness such as in your file. If you can provide steps, I can understand your frustration, but it does really help if you make them as simple and clear as possible. See #83679 for example

Fernando Alcala commented

2020-12-15 01:25:32 +01:00

Added subscriber: @Fernando-Alcala

Fernando Alcala commented

2020-12-15 01:25:32 +01:00

Blender's rigid body documentation is useless. But when you learn how to use it:
woke.mp4

Consistency can also be great. Here I have the original mass of 2kg where I change it to various other weights. You can clearly see the difference in behavior. Every time I go back to 2kg, it matches up perfectly.
constistancy.mp4

I also encountered an issue with Viewport render. After frame 3, some of the rigid bodies stopped moving, forcing me to use Workbench render. Even then I got 1 frame that I had to re-render.
broke.mp4

Blender's rigid body documentation is useless. But when you learn how to use it: [woke.mp4](https://archive.blender.org/developer/F9509123/woke.mp4) Consistency can also be great. Here I have the original mass of 2kg where I change it to various other weights. You can clearly see the difference in behavior. Every time I go back to 2kg, it matches up perfectly. [constistancy.mp4](https://archive.blender.org/developer/F9509136/constistancy.mp4) I also encountered an issue with Viewport render. After frame 3, some of the rigid bodies stopped moving, forcing me to use Workbench render. Even then I got 1 frame that I had to re-render. [broke.mp4](https://archive.blender.org/developer/F9509126/broke.mp4)

Jacques Lucke commented

2021-01-06 13:56:12 +01:00

Added subscriber: @JacquesLucke

Jacques Lucke commented

2021-01-06 13:56:12 +01:00

Is this a duplicate of #83679?

Jacques Lucke commented

2021-01-06 14:03:49 +01:00

Closed as duplicate of #82817

Jacques Lucke closed this issue

2021-01-06 14:03:49 +01:00

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Rigid Body and Cloth simulations are indeterministic and borderline unusable in production #83594