Simulation configuration

The simulation block declares the simulation backend and runtime parameters. The stereotype field decides which engine to use; UniSimFactory instantiates the matching UniSim subclass.

Source: fastsim/configs/simulation_cfg.py (SimulationConfig)

Common fields (all backends)

Field	Type	Description
`stereotype`	`str`	Backend identifier (e.g. `isaaclab` / `mujoco` / `pybullet`); must be registered in UniSim registry
`dt`	`float`	Physics timestep in seconds (must be > 0); typical `0.0166667` (60Hz)
`initialize_steps`	`int`	Init steps after reset to stabilize physics; default `60`
`num_envs`	`int`	Parallel environment count (GPU parallel simulation); default `1`
`env_spacing`	`float`	Spacing between parallel envs in meters; default `2.5`

List available registered backends:

bash

fastsim show-registry

IsaacLab backend

When stereotype: isaaclab, the config class is IsaacLabConfig (source: fastsim/simulators/isaaclab/isaaclab_config.py). Adds the following on top of common fields:

Field	Type	Description
`device`	`str`	Compute device, `cpu` or `cuda:<index>` (e.g. `cuda:0`)
`render_interval`	`int`	Render one frame every N physics steps; must be >= 1
`gravity`	`list[3]`	Gravity vector (x, y, z), default `[0.0, 0.0, -9.81]`
`physics_prim_path`	`str`	Physics scene prim path, default `/physicsScene`
`enable_scene_query_support`	`bool`	Enable scene queries (raycast etc.)
`use_fabric`	`bool`	Use Fabric data interface for accelerated access
`create_stage_in_memory`	`bool`	Create USD Stage in memory
`launch_config`	`LaunchConfig`	Isaac Sim AppLauncher params (incl. `headless`, `livestream`, `enable_cameras`, `device`)
`physics_config`	`PhysicsConfig`	PhysX physics engine parameters
`render_config`	`RenderConfig`	Rendering parameters (DLSS, shadows, etc.)

Configuration example

yaml

simulation:
  stereotype: isaaclab
  dt: 0.0166667
  initialize_steps: 60
  device: cuda:0
  render_interval: 1
  gravity: [0.0, 0.0, -9.81]
  create_stage_in_memory: true
  launch_config:
    headless: false
    livestream: 0
    enable_cameras: true
    device: cuda
  physics_config: {}
  render_config: {}

MuJoCo backend

When stereotype: mujoco, uses the MuJoCo physics engine. Lightweight CPU simulator, fast startup, suitable for quick iteration and debugging.

yaml

simulation:
  stereotype: mujoco
  dt: 0.002
  initialize_steps: 60

Source: fastsim/simulators/mujuco/mujuco_config.py

PyBullet backend

When stereotype: pybullet, uses the PyBullet physics engine — open-source CPU simulator with built-in OpenGL rendering.

yaml

simulation:
  stereotype: pybullet
  dt: 0.002
  initialize_steps: 60

Source: fastsim/simulators/pybullet/pybullet_config.py

Common errors

Error	Cause	Fix
`Simulation config is required`	Config file missing `simulation` block or `stereotype`	Add the complete `simulation` block
`Invalid simulator stereotype`	`stereotype` value not in registry	Run `fastsim show-registry` to check available values; for custom backends, ensure `scan_project` is enabled