What properties are used for physics simulation

The kit documentation simply says to look at tooltips for information on physics materials. There is some ambiguity here.

When making an empty physics material, below is what I see:

The youngs modulus is different in USD and Material. So which one is used?

Are MDL material information being used in physics?

What are the actual units? It says things like “mass” and “distance”, but these are not really specific SI units.

Are there any example materials that have been tested/validated to show they act as FEM material simulation is expected (foam, tennis ball, etc.)?

Furthermore, I can’t find information on the physics scene itself.

There is a “Bounce Threshold”

Which I can’t find in the API, and google search has literally one result:

Which is the IsaacSim paper. Besides it being on linux, it also lists a table with properties that are not in the menu, like “Restitution”

Hi,
The youngs modulus is different in USD and Material. So which one is used?

Its the same value, the physics material part is a special physics property widget showing the USD data in a nicer way. The raw data is just raw display of USD attributes.

Are MDL material information being used in physics?

If the material is assigned to the deformable body then yes, it should be used.

What are the actual units? It says things like “mass” and “distance”, but these are not really specific SI units.

No, the units depend on your USD stage units. There are USD metadata in your layer defining the units. So based on your units settings mass and distance is derived. Your stage can be setup in cm units, then all the input parameters has to be in cm. Mass is usually in kg. If you select the layer in layers window, then in the properties you will see metersPerUnit and kilogramsPerUnit values, those define mass and distance.

Are there any example materials that have been tested/validated to show they act as FEM material simulation is expected (foam, tennis ball, etc.)?

No, we dont provide such a library yet, but we are working on a way to provide it.

For the scene parameters, many of those advanced and detail parameters are exposing directly PhysX SDK parameters, see for bounce threshold:

Regards,
Ales

Hi @AlesBorovicka , Thanks for the explanations. I just wanted to double check some things.

You mention “Its the same value”, but as you can see in my screenshot, its not the same value.

So USD stage units change distance but not mass (re: “Mass is usually in kg”)?

Thanks for showing the line number. Actually I hadn’t been looking at physx4 because I was under the impression that the accurate FEM was only added in 5 (re: Announcing NVIDIA PhysX SDK 5.0 | NVIDIA Technical Blog)

Yes, the values can be different, reason is default values. Default values are in physics widget scaled by metersPerUnit in the raw data you see the raw default value, but thats not the value that is going to be used. The default value is dependent on metersPerUnit the physics widget knows that and it does display the default value in correct units.
kilogramsPerUnit can be changed in the layer metadata part, its just rare to see the value being different then 1.

Yes, Omniverse uses PhysX 5, but most of the parameters are still the same as for PhysX 4, so I did pointed to PhysX 4 documentation.

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