solid_and_soft.md

Physics (Flex)

Solid and soft actors

Solid actors

So far, we've seen how to add solid actors to a Flex scene. These will behave very similarly to default PhysX objects.

from tdw.controller import Controller
from tdw.tdw_utils import TDWUtils

c = Controller()
object_id = c.get_unique_id()
c.communicate([TDWUtils.create_empty_room(12, 12),
               {"$type": "convexify_proc_gen_room"},
               {"$type": "create_flex_container"},
               c.get_add_object(model_name="cube",
                                object_id=object_id,
                                library="models_flex.json",
                                position={"x": 0, "y": 1, "z": 0}),
               {"$type": "set_flex_solid_actor",
                "id": object_id,
                "mesh_expansion": 0,
                "mass_scale": 1,
                "particle_spacing": 0.05},
               {"$type": "assign_flex_container",
                "id": object_id,
                "container_id": 0}])
c.communicate({"$type": "terminate"})

• mesh_expansion will resize the mesh. This should almost always be 0.
• particle_spacing is the distance between each particle. This typically needs to be adjusted depending on the geometry and vertices of the underlying mesh data.
• mass_scale should match the mass you'd normally assign a PhysX object.

Soft actors

Enable an object as a soft-body actor via set_flex_soft_actor:

from tdw.controller import Controller
from tdw.tdw_utils import TDWUtils
from tdw.add_ons.third_person_camera import ThirdPersonCamera
from tdw.add_ons.image_capture import ImageCapture
from tdw.backend.paths import EXAMPLE_CONTROLLER_OUTPUT_PATH

c = Controller()
camera = ThirdPersonCamera(avatar_id="a",
                           position={"x": 3.83, "y": 3.6, "z": -0.71},
                           look_at={"x": 0, "y": 0, "z": 0})
path = EXAMPLE_CONTROLLER_OUTPUT_PATH.joinpath("flex_soft_body")
print(f"Images will be saved to: {path}")
capture = ImageCapture(avatar_ids=["a"], path=path)
c.add_ons.extend([camera, capture])
cube_id = c.get_unique_id()
dumbbell_id = c.get_unique_id()
c.communicate([TDWUtils.create_empty_room(12, 12),
               {'$type': 'convexify_proc_gen_room'},
               {'$type': 'create_flex_container'},
               c.get_add_object(model_name="cube",
                                object_id=cube_id,
                                library="models_flex.json",
                                position={"x": 0, "y": 0, "z": 0}),
               {"$type": "set_flex_solid_actor",
                "id": cube_id,
                "mass_scale": 50,
                "particle_spacing": 0.125},
               {"$type": "assign_flex_container",
                "id": cube_id,
                "container_id": 0},
               c.get_add_object(model_name="dumbbell",
                                object_id=dumbbell_id,
                                library="models_flex.json",
                                position={"x": 0.25, "y": 2, "z": 0}),
               {"$type": "set_flex_soft_actor",
                "id": dumbbell_id,
                'particle_spacing': 0.05,
                'cluster_stiffness': 0.5,
                "mass_scale": 1},
               {"$type": "assign_flex_container",
                "id": dumbbell_id,
                "container_id": 0}])
for i in range(200):
    c.communicate([])
c.communicate({"$type": "terminate"})

Result:

Soft actor parameters

There are many soft actor parameters; see the API documentation for more information. Unfortunately, most of descriptions of the parameters are uninformative because they aren't explained in the NVIDIA Flex documentation.

Of these, the most important (besides id, of course) is particle spacing. If the particles are too far apart, the object will appear to tear. If the particles are too close, the simulation can dramatically slow down. The ideal particle spacing varies by object and can only be determined via trial and error.

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Next: Cloth actors

Return to the README

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Example controllers:

• soft_body.py Minimal soft body simulation.

Command API:

• set_flex_solid_actor
• set_flex_soft_actor