Every Obi simulation requires at least one solver. In TDW, the Obi add-on will automatically send create_obi_solver when it initializes or resets (see below). In most cases, this automatically-created solver will be sufficient for your simulation.
Each solver has an integer ID corresponding to the order in which they were created: The first solver is 0, the second is 1, and so on.
Whenever you add actors such as fluids, they must be assigned a solver. This is handled by a solver_id parameter; for example, obi.create_fluid() has an optional solver_id parameter (default value is 0).
You can send create_obi_solver more than once to create additional solvers. There are two main reasons to do this:
- Actors assigned to different solvers won't interact with each other.
- Solvers can have different parameters, which may be needed for different Obi actors.
Set solver parameters by calling obi.set_solver() which will send set_obi_solver_substeps and set_obi_solver_scale.
- Substeps are sub-frames between
communicate()calls. More substeps can greatly increase the accuracy of the simulation at the cost of speed. They can also prevent tearing glitches with cloth. - The scale of a solver will uniformly scale all of its actors.
In this example, we'll define a highly viscous fluid that looks and behaves like strawberry jam. We'll increase the number of solver substeps from 1 to 4 in order to allow the viscous jam to behave correctly, and we'll make the floor and reference object sticky.
from tdw.controller import Controller
from tdw.add_ons.obi import Obi
from tdw.add_ons.third_person_camera import ThirdPersonCamera
from tdw.obi_data.fluids.disk_emitter import DiskEmitter
from tdw.obi_data.fluids.fluid import Fluid
from tdw.obi_data.collision_materials.collision_material import CollisionMaterial
from tdw.obi_data.collision_materials.material_combine_mode import MaterialCombineMode
c = Controller()
c.communicate(Controller.get_add_scene(scene_name="tdw_room"))
fluid_id = Controller.get_unique_id()
object_id = Controller.get_unique_id()
camera = ThirdPersonCamera(position={"x": -3.75, "y": 1.5, "z": -0.5},
look_at={"x": 0, "y": 0, "z": 0})
# Define a sticky collision material.
sticky_material = CollisionMaterial(dynamic_friction=0.8,
static_friction=0.8,
stickiness=0.9,
stick_distance=0.1,
stickiness_combine=MaterialCombineMode.average,
friction_combine=MaterialCombineMode.average)
# Define a custom fluid.
fluid = Fluid(capacity=1500,
resolution=0.75,
color={"r": 1.0, "g": 0.1, "b": 0.1, "a": 1.0},
rest_density=1000,
reflection=0.25,
refraction=0.05,
smoothing=2.5,
viscosity=2.5,
vorticity=0,
surface_tension=1,
transparency=0.85,
radius_scale=2.0)
# Initialize Obi.
obi = Obi(floor_material=sticky_material, object_materials={object_id: sticky_material})
c.add_ons.extend([camera, obi])
# Increase the solver substeps to accommodate the high viscosity and smoothing and make the fluid behave more like jam.
obi.set_solver(substeps=4)
# Create a disk-shaped emitter, pointing straight down.
obi.create_fluid(fluid=fluid,
shape=DiskEmitter(radius=0.2),
object_id=fluid_id,
position={"x": -0.1, "y": 2.0, "z": 0},
rotation={"x": 90, "y": 0, "z": 0},
lifespan=20,
speed=2)
# Add an object for the fluid to interact with.
c.communicate(Controller.get_add_physics_object(model_name="sphere",
object_id=object_id,
library="models_flex.json",
kinematic=True,
gravity=False,
scale_factor={"x": 0.5, "y": 0.5, "z": 0.5}))
for i in range(500):
c.communicate([])
c.communicate({"$type": "terminate"})Result:
Cloth sheets can't be scaled like cloth volumes can; this is because scaling the sheet would affect the data used for tethering. Instead, you can scale a cloth sheet by scaling the solver.
In this example, we'll create a cloth sheet, tether it to an object, scale the solver, and rotate the object:
from tdw.controller import Controller
from tdw.add_ons.obi import Obi
from tdw.add_ons.third_person_camera import ThirdPersonCamera
from tdw.obi_data.cloth.tether_particle_group import TetherParticleGroup
from tdw.obi_data.cloth.tether_type import TetherType
from tdw.obi_data.cloth.sheet_type import SheetType
c = Controller()
c.communicate(Controller.get_add_scene(scene_name="tdw_room"))
camera = ThirdPersonCamera(position={"x": -3.75, "y": 1.5, "z": -0.5},
look_at={"x": 0, "y": 1.25, "z": 0})
obi = Obi()
c.add_ons.extend([camera, obi])
cloth_id = Controller.get_unique_id()
cube_id = Controller.get_unique_id()
# Scale the solver.
obi.set_solver(scale_factor=0.75,
substeps=2)
# Create a sheet that looks and behaves like canvas, that we will attach to a bar-shaped object.
# Note the offset in Z, required to line up the "north" edge of the sheet with the object.
obi.create_cloth_sheet(cloth_material="canvas",
object_id=cloth_id,
position={"x": 0, "y": 2.0, "z": -1.0},
rotation={"x": 0, "y": 0, "z": 0},
sheet_type=SheetType.cloth_hd,
tether_positions={TetherParticleGroup.north_edge: TetherType(cube_id)})
# Create the long bar-shaped attachment object.
c.communicate(Controller.get_add_physics_object(model_name="cube",
object_id=cube_id,
library="models_flex.json",
position={"x": 0, "y": 2.0, "z": 0},
kinematic=True,
gravity=False,
scale_factor={"x": 3.0, "y": 0.1, "z": 0.1}))
# Let the cloth object settle.
for i in range(150):
c.communicate([])
# Rotate the bar back and forth, moving the cloth with it.
for i in range(480):
c.communicate({"$type": "rotate_object_by",
"id": cube_id,
"axis": "yaw",
"is_world": False,
"angle": 1})
for i in range(540):
c.communicate({"$type": "rotate_object_by",
"id": cube_id,
"axis": "yaw",
"is_world": False,
"angle": -1})
c.communicate({"$type": "terminate"})Result:
When resetting the scene for a new trial i.e. when obi.reset() is called, the Obi add-on sends destroy_obi_solver to destroy the initial solver.
Send additional destroy_obi_solver commands to clean up any other solvers that you created.
Next: Robots and Obi
Example controllers:
- strawberry_jam.py Create a custom "strawberry jam" fluid, and a custom collision material.
- tether_object.py Tether a cloth sheet to another object and scale the sheet via the solver.
Python API:
Command API: