Cloth collision with Alembic mesh not working.

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I've imported an Alembic character sequence from Softimage (with point velocities exported) and created a shirt cloth object.
Collision does not work and I presume it is because it is a sequence of meshes with no velocity data on them? Not sure why Houdini can't see/read the exported point velocities.

What would be the right way to get this to work?
Werner Ziemerink
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1. you can check the collition mesh if its correct:
dop network => collision geometry (RBD Object node) => Collitions (tab: RBD Solver) => Collition Guide (tab: volume)
make the viewport to wireframe

2. add the ability to generate on each frame:
dop network => collision geometry (RBD Object node) => Use Deforming Geometry

i think one of these two are the cause of the problem but not 100% sure.

some tutorials:
https://www.sidefx.com/index.php?option=com_content&task=view&id=2622&Itemid=344 [sidefx.com]
https://www.sidefx.com/index.php?option=com_content&task=view&id=2350&Itemid=344 [sidefx.com]
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Nevermind. I found a workaround.

If anyone is interested…

- Export just first frame of your character mesh as obj or alembic.
- Use this static mesh as the collider and append a point SOP after the file node.
- Use the alembic sequence as the second input of the point SOP and set the expression to $TX2 $TY2 and $TZ2

Done!

Thanks mandrake. I will have a look and let you know if it works.
Werner Ziemerink
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Thx Werner, you just saved my life!
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Hi Werner,

I don't know how to use the alembic sequence as the second input of the point SOP and set the expression to $TX2 $TY2 and $TZ2, in what expression? could you please help me out?

Thank you
SICONG
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Are u sure you just didn't forget to unpack/convert your alembic?
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Werner Ziemerink
Nevermind. I found a workaround.

If anyone is interested...

- Export just first frame of your character mesh as obj or alembic.
- Use this static mesh as the collider and append a point SOP after the file node.
- Use the alembic sequence as the second input of the point SOP and set the expression to $TX2 $TY2 and $TZ2

Done!

Thanks mandrake. I will have a look and let you know if it works.

Hello, I am quite late to the party but I have I think a similar issue.

I drapped some kind of dress around a Mixamo model. It works fine but then, when I start my sim and my model starts walking, the dress doesn't collide at all. I realised it was because I edited the model and the topology is changing every frame. I mean the points of the model are changing each frame. The mesh isn't really static. I guess it happened because I inflated some body parts with VDBs and now the arms are merging with the hips as the model is walking, but I want to keep that effect.

So I thought that was the cause of my collisions problems. I tried with a normal Mixamo sequence and I didn't have any issues. So It definitely is because of the changing topology of my model.

It seems that the solution would be to convert my walking model into a collision VDB object (correct me if I am wrong) and do this in DOPS, import the drape as vellum source. But it still doesn't work
My static object in DOPS (my walking model) is set to:

- use deforming geometry
- create active object (tried both on and off)
- collisions detection set to volume (I also tried surface)
- Volume sample mode

It is not working. The cloth is kind of colliding, but intersect with my model and it is really jittery.
I have increased substeps with no change.

Any clues?? I tried what you said with the Point SOP, but my mesh doesn't move after putting this expression in it.

Thanks a lot
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I'm not sure vellum works with volume collisions. It has its own way of computing collisions based on the mesh since it's pbd.

Use an animation that doesn't have changing topology. It's very abnormal for skeletal animation to wind up with changing topology. I would retrace my steps to try to resolve that problem before doing anything. Changing topology makes it impossible to compute velocities or motion blur.
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jsmack
I'm not sure vellum works with volume collisions. It has its own way of computing collisions based on the mesh since it's pbd.

Use an animation that doesn't have changing topology. It's very abnormal for skeletal animation to wind up with changing topology. I would retrace my steps to try to resolve that problem before doing anything. Changing topology makes it impossible to compute velocities or motion blur.

Thanks for your answer. Ok, so the problem comes from Vellum then.
What would be the process to make a cloth collide with a deforming VDB? I tried with a mesh that doesn't have a changing topology and it works fine but I would like to find a solution for something that is changing topology.
Edited by anbt - Jan. 21, 2021 14:59:49
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I think the FEM based cloth solver supports volume collisions, but it's a lot slower than vellum.
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jsmack
I think the FEM based cloth solver supports volume collisions, but it's a lot slower than vellum.
ok never heard about FEM yet, I will try to see if I can find a tutorial. Thanks
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Vellum supports volume collisions, but just points will collide, which I guess it's similar for other solvers
Tomas Slancik
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Have you also looked at this thread? https://www.sidefx.com/forum/topic/77472/ [www.sidefx.com]
Sean Lewkiw
Lost Boys Montreal
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mrCatfish
Have you also looked at this thread? https://www.sidefx.com/forum/topic/77472/ [www.sidefx.com]
yes this is also me in this thread
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