Scene

Scene is the declarative description of the simulation world — you specify entities, their types, and asset sources in YAML, and SceneManager converts this into interactive simulation entities at runtime.

Entity Types

Category	Config Key	Stereotype	Description
Base	`base_config`	`ground_plane` / `usd` / `empty`	Scene foundation, one per scene
Object	`object_cfg_dict`	`rigid` / `articulated` / `soft`	Rigid bodies, articulated objects, or deformable bodies
Robot	`robot_cfg_dict`	`general_robot`	Robots with modular composition and planner support
Sensor	`sensor_cfg_dict`	`camera`	Sensors, primarily cameras
Light	`light_cfg_dict`	`general_light`	Light sources

Modular Robot

FastSim uses a unified general_robot stereotype with modular composition via modules to describe robot structure — including base, arms, end-effectors, and extras. This supports single-arm, dual-arm, and more complex robot configurations.

yaml

robot_cfg_dict:
  my_robot:
    stereotype: general_robot
    asset_path: assets://robots/franka.usd
    use_planner: true
    modules:
      arms:
        main_arm:
          actuator_name: panda_arm
          ee_link_name: panda_hand
      end_effectors:
        gripper:
          actuator_name: panda_gripper

Each arm module supports independent ee_link_name and ik_joint_names, and can be configured with independent planners for multi-arm robots.

Update Rhythm

Each frame of FastSim.step() follows a strict three-phase pattern:

text

① SceneManager.pre_physics_update()  ← Write target values to simulator
② UniSim.step()                      ← Physics advances one frame
③ SceneManager.post_physics_update() ← Read latest state from simulator

Asset Path Resolution

Format	Example	Description
Absolute	`/data/robots/ur5.usd`	Direct filesystem path
Relative	`./assets/cube.usd`	Relative to config file directory
root_key://	`assets://robots/ur5.usd`	Maps to `general.root_paths.assets`
HTTP(S)	`https://example.com/ur5.usd`	Remote asset, auto-downloaded and cached