Hello, DART

Let’s write the smallest useful DART program: drop a box onto the ground and watch it fall. Every DART simulation follows the same three beats — build a world, lock it, step it — and this example shows all three.

The full program

import dartpy as dart
import numpy as np

# 1. Create a world. The time step is the physics integration interval.
world = dart.World(time_step=1.0 / 1000.0)
world.gravity = np.array([0.0, 0.0, -9.81])

# 2. Add a static ground plate. CollisionShape.box takes *half* extents, so this
#    is a 4 m x 4 m x 0.1 m slab; placing its center at z = -0.05 puts the top
#    surface at z = 0.
ground_opts = dart.RigidBodyOptions()
ground_opts.is_static = True
ground_opts.position = np.array([0.0, 0.0, -0.05])
ground = world.add_rigid_body("ground", ground_opts)
ground.set_collision_shape(dart.CollisionShape.box(np.array([2.0, 2.0, 0.05])))

# 3. Add a 1 kg box one metre above the ground.
box_opts = dart.RigidBodyOptions()
box_opts.mass = 1.0
box_opts.position = np.array([0.0, 0.0, 1.0])
box = world.add_rigid_body("box", box_opts)
box.set_collision_shape(dart.CollisionShape.box(np.array([0.1, 0.1, 0.1])))

# 4. Lock the topology, then advance the simulation.
world.enter_simulation_mode()
for step in range(1000):
    world.step()
    if step % 100 == 0:
        height = float(box.translation[2])
        print(f"t = {world.time:5.3f} s   box height = {height:6.4f} m")

Running it prints the box falling and settling on the ground:

t = 0.000 s   box height = 1.0000 m
t = 0.100 s   box height = 0.9519 m
t = 0.200 s   box height = 0.8076 m
t = 0.300 s   box height = 0.5670 m
t = 0.400 s   box height = 0.2303 m
t = 0.500 s   box height = 0.1000 m
...

(The exact numbers depend on your DART version and solver settings; the shape of the motion — accelerate, then rest on the ground — is what matters.)

What each step does

Build a world. dart.World is the single object that owns everything: bodies, time, gravity, and the step pipeline. The time_step is how much simulated time each step() advances.
Add bodies. A RigidBodyOptions value object carries the initial state (mass, pose, velocity, whether the body is static). add_rigid_body returns a handle you keep to read or change the body later. A static body never moves — perfect for ground and walls. Giving a body a collision shape lets it take part in contact.
Lock the topology. enter_simulation_mode() finalizes the scene so DART can allocate runtime state. After this point you step rather than restructure; see The World for the design-mode / simulation-mode split.
Step. Each world.step() integrates the physics forward by one time_step and refreshes every body’s state. Reading box.translation afterwards returns the fresh world-frame position.

Things to try

Change box_opts.position to start the box off-center and watch it topple as it lands.
Give the box an initial spin with box_opts.angular_velocity = np.array([0.0, 4.0, 0.0]).
Add a second box with a different name and mass, then print both heights.

Next

You have a running simulation. Next, look more closely at the loop that drives it in The simulation loop.