DART 7 · in development You are reading the documentation for DART 7, an in-progress redesign that is not yet recommended for production use. Looking for the current stable release? See the DART 6 LTS documentation.

The World

dart.World is the heart of DART 7. A single World object owns your scene’s topology (bodies, multibodies, and the connections between them), the simulation clock, gravity, and the step pipeline that advances physics. Almost everything you do in DART starts from a world.

Creating a world

You can create a world with defaults and configure it later, or pass options up front:

import dartpy as dart

# Defaults
world = dart.World()

# Configured at construction
world = dart.World(
    time_step=1.0 / 1000.0,
    rigid_body_solver=dart.RigidBodySolver.SEQUENTIAL_IMPULSE,
    contact_solver_method=dart.ContactSolverMethod.SEQUENTIAL_IMPULSE,
)

Solver choices are covered in Choosing solvers.

Two modes: design, then simulate

A DART world has two phases, and keeping them separate is what makes stepping fast and predictable:

• Design mode (the default after construction) is where you build the scene: add rigid bodies, add multibodies, attach shapes, and set initial state.

• Simulation mode is where you run it. Calling enter_simulation_mode() finalizes the topology so DART can allocate runtime state once, up front.

# --- design mode: build the scene ---
ground = world.add_rigid_body("ground", ground_opts)
robot = world.add_multibody("arm")
# ...

# --- switch to simulation mode, then step ---
world.enter_simulation_mode()
world.step()

world.step() will enter simulation mode for you on the first call, so the explicit call is mainly there when you want deterministic allocation or to catch topology errors at a known point. After finalization, restructuring the scene requires an explicit reset or rebuild rather than ad-hoc edits — this is what lets the per-step path stay allocation-free.

What a step does

world.step() runs a content-aware pipeline: it executes only the stages your scene needs. A scene of free rigid bodies runs rigid-body integration and contact resolution; add an articulated robot and the multibody dynamics stage joins in; the pipeline finishes with a kinematics refresh so every transform you read afterwards is current. You select capabilities and policies (which solver, which contact method) rather than wiring stages together by hand.

Kinematics without dynamics

Sometimes you only need to update where things are — for motion planning, collision queries, playback, or visualization — without integrating forces. Set the poses you care about and refresh the frames:

world.enter_simulation_mode()
# ... set joint or body positions ...
world.update_kinematics()     # refresh transforms; no force integration

This reuses the same world, bodies, and frames as full physics, so you never have to maintain a separate “kinematics-only” copy of your scene.

Inspecting the world

A world exposes simple lookups and counters for the objects it owns, plus the clock you read in the loop:

world.time            # current simulated time (seconds)
world.time_step       # integration interval
world.collide()       # run collision detection now; returns the contacts

Next

With the world understood, the next two pages cover the things you put inside it: rigid bodies and articulated systems.