Extract mesh vertices and edges from inflatable simulation demo

[jeffreybliang]

Hi everyone,
In USD Composer, there is an inflatable simulation tutorial on the PhysX documentation. I would like to edit the simulation script (shown below) so that at each timestep of the simulation, the vertices and edges (if there are any) of the cloth/mesh are written to a file - this way I can reconstruct the mesh of the inflatable.
I am aware there exists PxParticleBuffer::getPositionInvMasses()/ PxParticleBuffer::getVelocities() to get the position and velocities of particles, but I'm unsure on how this would be called in the Python script below. Specifically, how to obtain these in Python at every time step.

Does anyone know how to do so? I imagine it would be like a callback function that executes with each timestep but unsure.
Thanks!

import carb
import math
from omni.physx.scripts import physicsUtils, particleUtils
from pxr import UsdGeom, Sdf, Gf, PhysxSchema, UsdPhysics
import omni.usd
import omni.physxdemos as demo
from .ParticleDemoBaseDemo import ParticleDemoBase


class ParticleInflatableDemo(demo.Base, ParticleDemoBase):
    title = "Particle Inflatable"
    category = demo.Categories.PARTICLES
    short_description = "Basic demo showing setup of a sphere as a particle inflatable"
    description = "A basic sphere mesh is setup as a particle simulation inflatable cloth mesh."

    def create(self, stage):
        default_prim_path = self.setup_base_scene(stage, zoomAmount = 0.8)

        # configure and create particle system
        # we use all defaults, so the particle contact offset will be 5cm / 0.05m
        # so the simulation determines the other offsets from the particle contact offset
        particle_system_path = default_prim_path.AppendChild("particleSystem")
        particle_system = PhysxSchema.PhysxParticleSystem.Define(stage, particle_system_path)
        particle_system.CreateSimulationOwnerRel().SetTargets([self._scene.GetPath()])

        # create material and assign it to the system:
        particle_material_path = default_prim_path.AppendChild("particleMaterial")
        particleUtils.add_pbd_particle_material(stage, particle_material_path)
        physicsUtils.add_physics_material_to_prim(
            stage, stage.GetPrimAtPath(particle_system_path), particle_material_path
        )

        # create a mesh that is turned into an inflatable
        cube_u_resolution = 20
        cube_v_resolution = 20
        cube_w_resolution = 4

        success, tmp_path = omni.kit.commands.execute(
            "CreateMeshPrimWithDefaultXform",
            prim_type="Cube",
            u_patches=cube_u_resolution,
            v_patches=cube_v_resolution,
            w_patches=cube_w_resolution,
            u_verts_scale=1,
            v_verts_scale=1,
            w_verts_scale=1,
            half_scale=60.0,
        )

        cloth_mesh_path = Sdf.Path(omni.usd.get_stage_next_free_path(stage, "/Inflatable", True))
        omni.kit.commands.execute("MovePrim", path_from=tmp_path, path_to=cloth_mesh_path)

        cloth_mesh = UsdGeom.Mesh.Get(stage, cloth_mesh_path)
        physicsUtils.setup_transform_as_scale_orient_translate(cloth_mesh)
        physicsUtils.set_or_add_translate_op(cloth_mesh, Gf.Vec3f(0.0, 200.0, -20.0))
        physicsUtils.set_or_add_orient_op(cloth_mesh, Gf.Quatf(Gf.Rotation(Gf.Vec3d(1,0,0), 10.0).GetQuat()))
        physicsUtils.set_or_add_scale_op(cloth_mesh, Gf.Vec3f(1.0, 1.0, 0.04))

        # configure as cloth
        pressure = 8.0
        stretchStiffness = 20000.0
        bendStiffness = 100.0
        shearStiffness = 100.0
        damping = 0.5
        particleUtils.add_physx_particle_cloth(
            stage=stage,
            path=cloth_mesh_path,
            dynamic_mesh_path=None,
            particle_system_path=particle_system_path,
            spring_stretch_stiffness=stretchStiffness,
            spring_bend_stiffness=bendStiffness,
            spring_shear_stiffness=shearStiffness,
            spring_damping=damping,
            pressure=pressure,
        )

        # configure mass:
        particle_mass = 0.1
        num_verts = len(cloth_mesh.GetPointsAttr().Get())
        mass = particle_mass * num_verts
        massApi = UsdPhysics.MassAPI.Apply(cloth_mesh.GetPrim())
        massApi.GetMassAttr().Set(mass)

        # add render material:
        material_path = self.create_pbd_material("OmniPBR")
        omni.kit.commands.execute(
            "BindMaterialCommand", prim_path=cloth_mesh_path, material_path=material_path, strength=None
        )

[vreutskyy]

Hi @jeffreybliang
It may be better to ask on some Omniverse / Omni Physics forum, not PhysX SDK repo
A quick search gave me this from Omni Physics docs, not sure if it's what you're looking for though.
https://docs.omniverse.nvidia.com/kit/docs/omni_physics/latest/extensions/runtime/source/omni.physics.tensors/docs/api/python.html#omni.physics.tensors.impl.api.ParticleClothView.get_positions
Cheers

[jeffreybliang]

Hi @vreutskyy
Thank you for pointing me to that page. That is one step in the right direction.
I think the main priority would be to figure out how to call that each step still.
I'll check out the other forum, thanks!