[Bug] Intersection / self-intersection failures on animated cape cloth

[ryichando]

Summary

Simulating cloth on a cape attached to an animated character fails repeatedly with intersection or self-intersection errors. The solver breaks after roughly 6 frames, though the exact frame varies with the animation used. Halving the substep count did not help.

Steps to Reproduce

1. Open the reproduction blend file from the discussion attachment (blend is inside the zip since GitHub does not accept .blend).
2. The scene has a character with an armature animation and a cape set up as cloth. The animation starts from a resting A-pose and fades into the main motion, intended to give the cloth a clean start.
3. Run the simulation from frame 1.
4. Observe the failure around frame 6.

Reproduction files from the discussion:

• Blend file (zip): https://github.com/user-attachments/files/30083406/batman.zozo.issue.zip
• Mirror: https://www.mediafire.com/file/4g8ma3z319bdswg/batman_zozo_issue.blend/file
• Intersection records from a failing run: https://github.com/user-attachments/files/30083270/intersection_records.json

The reporter recreated the issue with a permissively licensed model: https://www.nexusmods.com/batmanarkhamknight/mods/624?tab=description

Expected Behavior

The cape simulates through the full animation without intersection or self-intersection errors.

Actual Behavior

The solver stops with intersection / self-intersection errors, typically around frame 6. Reducing substeps to half did not change the outcome. Starting from an A-pose and fading in the animation did not help either.

Environment

• Add-on version: Unknown
• Solver version: Unknown
• Blender version: Unknown
• OS/GPU/driver: Unknown

Contribution Agreement

• I confirm that any code, patches, scripts, attachments, or other copyrightable material I include in this issue may be used, reproduced, modified, and distributed by this project under the Apache License 2.0 or as otherwise described in CONTRIBUTING.md.
• This is a bug report from the community (please provide a reference below).

Community Reference

#109

[amourakyn]

i have somewhat identified the issue, pinning vertices are exempt from the cloth simulation so when the body goes to interact with those vertices they intersect and cause an error in the simulation. the simulation works fine with a cropped version of the collider + less pinning or no pinning at all and keeping the collider mesh the same.

this is still an issue for me though since i'd want the cape to conform to the shoulders/clavicles without simulating too heavily but also still have the rest of the cape react normally.

i've browsed the documentation briefly but i don't think ive seen any mention of a feature like this, where the cloth can attract and conform to something for a group of vertices.

here's a good example of what i'm talking about, at 9:18 you can see the cape conform to batmans shoulders whilst still being under the shoulder pads/ armor. if i were to do my "no pin fix" it would clip through the shoulder pads in certain animations

(you may be wondering why i don't add the shoulder pads as a collider but this is because i wasn't able to find an optimal placement for the cloth simulation to work since the models are intersecting, i have tried making some changes in the geometry but it's not entirely ideal and also in other suits without the shoulder pads i'd still like parts of the cape to conform to the body)

https://youtu.be/RJBVla8s9Q4

[ryichando]

I think I was able to reproduce the issue as attached:

zozo-batman-issue-50.mp4

I will start looking into the issue, but the first thing I discovered is that the character animation in this file is too fast for our physics simulation to handle within the single precision, but not 100% sure that is the problem.

[amourakyn]

problem.

instead of using the nla editor i went into the main animation in pose mode, moved all the key frames ahead by 30 frames, then on the first frame reset the rotation and position so it has enough time to have a clean sim, i commented my other theories on why this issue could be happening i think it may be of some help.

[ryichando]

@amourakyn Thank you! Do you mind uploading the fixed .blend file so that we all can precisely share the possible fix you suggested? You can zip the .blend file and drag-and-drop here.

[amourakyn]

batman zozo test.zip

heres the file with my "fixes" applied (+ raising cape above batmans body only, original position is at 0,0,0) once again this simulates fine but the moment i add pins theres issues since the bone animation and the pinned vertices of the cape overlap. my current fix isnt ideal since i want the top parts of the cape to conform to batmans shoulders so they dont bounce around. for the model the top part of the cape is meant to sit under the shoulderpads so im wondering if theres a way to make a cloth simulation be attracted to a group of vertices/ a shape in general.

[ryichando]

Thanks, I confirm the attached .blend file perfectly works, see the attached

batman-anim-fixed-suggested-50.mp4

but the animation initially uploaded looks quite different. See the attached

batman-attached-original-50.mp4

[amourakyn]

Thanks, I confirm the attached .blend file perfectly works, see the attached

batman-anim-fixed-suggested-50.mp4

but the animation initially uploaded looks quite different. See the attached

batman-attached-original-50.mp4

apologies i ended up using a different animation for that blend scene, the issues are still present in either without my fixes though.

as for the animations being "too fast" im sure i could make a work around for the cloth to simulate slower and to slow down the animation too.

[ryichando]

OK, then is everything fine now? If something is still bothering you I can try make fixes.

[ryichando]

heres the file with my "fixes" applied (+ raising cape above batmans body only, original position is at 0,0,0) once again this simulates fine but the moment i add pins theres issues since the bone animation and the pinned vertices of the cape overlap. my current fix isnt ideal since i want the top parts of the cape to conform to batmans shoulders so they dont bounce around. for the model the top part of the cape is meant to sit under the shoulderpads so im wondering if theres a way to make a cloth simulation be attracted to a group of vertices/ a shape in general.

Maybe this is you hope to be fixed. Let me look into it.

[amourakyn]

OK, then is everything fine now? If something is still bothering you I can try make fixes.

sadly still issues, my fix isn't an ideal fix since i'm only simulating the cape to fall ontop of batmans body for the initial 30 frames. it appears fine but the cape of batman itself is meant to be under his shoulder pads, and if i were to remove the shoulder pads from purely the static group only then the cape would clip through and be visible through the shoulders at parts of the animation. it's not something i can pin either since pinning will cause errors.

my main question now is if there's a way for the cloth to take on the shape of the shoulders for batman and rest below the shoulder pads whilst still simulating the rest of the cape.

https://youtu.be/RJBVla8s9Q4?t=556

9:17 would be a good example, the cape is resting under the shoulders whilst the rest of the cape still animates

let me know if i'm explaining the issue properly.

typically i wouldnt mind this issue but this isn't something i can ignore since theres animations with batman doing jumps etc and even just in general the general top area should conform to batmans shoulders.

[ryichando]

OK, I will work on it.