Skipping attachment creation

[garciaguevara]

Hello, I have an error when using the deformable body attachments in the last Isaac Sim 2023.1.0-hotfix.1 release.

It can be reproduced with the extsPhysics/omni.physx.demos-105.1.10-5.1/omni/physxdemos/scenes/DeformableBodyAttachmentsDemo.py. If we set the left blob deformable body simulation_hexahedral_resolution to 64 (it can also be changed in the Mesh/Physcs/Deformable Body UI).

deformableUtils.add_physx_deformable_body(stage, blob_left_mesh.GetPath(), collision_simplification=False, simulation_hexahedral_resolution=64)

This change sets more attachment points but when the simulation starts the attachment is not properly created. The left blob is actually detached from the cube, showing this error:

[Error] [omni.physx.plugin] Invalid attachment point detected for attachment /blobLeft/attachment. Skipping attachment creation.

I think, this issue is not related to simulation_hexahedral_resolution but it is due to a limit on the number of attachment points that can be handled by PhysxSchema.PhysxPhysicsAttachment. As another example, a low resolution deformable body attached into a large overlap object (more attachment points) causes the same error.

I tested this with the previous 2022.2.1 release and it did not have this issue, it maintained correctly attached the blob, independently of the number of attached points.

Best

[clow]

I followed your steps using Isaac Sim 2023.1.0-hotfix. 1 release but unfortunately, I was unable to reproduce the bug. Could you please verify again from a clean installation?

[clow]

Another thing I found unusual is that from your attached screenshots, the number of visual tets on the left blob has increased significantly. If you only changed simulation_hexahedral_resolution, the number of visual tets should not increase as only the simulation tets are affected. Increasing the number of simulation tets will make the left blob much more stretchable than the right blob but will not change the number of visual tets.

[garciaguevara]

Hello,

You are completely right, I was testing with the old example version. In any case, the bug can be reproduced as well with the 2023 sample.

One need to set the collision_simplification=True (which, as you mention, adapts the number of collision tets accordingly to the deformable resolution) and the simulation_hexahedral_resolution=92 in the line 47 of the ~/.local/share/ov/pkg/isaac_sim-2023.1.0-hotfix.1/extsPhysics/omni.physx.demos-105.1.10-5.1/omni/physxdemos/scenes/DeformableBodyAttachmentsDemo.py example.

deformableUtils.add_physx_deformable_body(stage, blob_left_mesh.GetPath(),
collision_simplification=True, simulation_hexahedral_resolution=92)

I know doing this is quite an overkill for this toy example, but in a real case that I am working on is very useful to do a collision simplification and moreover the same bug repeats even with coarse tets when the attachment contact is large, which is again very useful if one wants to attach a large part of a body. This was working in version 2022 and seems to be limited in the 2023 version by the number of attached points.

Thanks a lot for your advice!

[clow]

I can reproduce the issue you are seeing. This bug has been fixed on the newer physics extension but the changes have to be incorporated into Isaac Sim. I will file a bug report to the Isaac team so that they can address this issue.

[garciaguevara]

Thanks a lot! Do you know if there might be a way to use the fixed physics extension without waiting for its Isaac Sim incorporation?

[SimonPhysX]

Sorry, I don’t think there is going to be a separate physics extension update before Isaac Sim will have updated.

[garciaguevara]

Thanks anyways I will wait for Isaac sim update

[SimonPhysX]

Hi @garciaguevara ,
we confirmed again, this issue should be fixed with the next Isaac Sim release.