# Backends

FIGAROH's algorithms (regressor construction, calibration, identification) are written once against an abstract `DynamicsBackend` interface, not against a specific simulator. Swapping the simulator underneath a workflow is a config change, not a rewrite.

## Available backends

| Backend       | Formats         | Status                                 | Best for                                      |
| ------------- | --------------- | -------------------------------------- | --------------------------------------------- |
| **Pinocchio** | URDF            | Always available (core dependency)     | Research & identification, CPU                |
| **MuJoCo**    | URDF, MJCF      | Optional (`pip install mujoco`)        | Contact dynamics, optimal control, sparse ops |
| **Genesis**   | URDF, MJCF, USD | Optional (`pip install genesis-world`) | GPU, large-scale parallel/multi-robot         |
| **Isaac Sim** | USD, URDF       | Optional, requires NVIDIA Omniverse    | Sim-to-real, RL, synthetic data               |

```
from figaroh.backends import get_backend, list_backends

print(list_backends())
# {'pinocchio': True, 'mujoco': True, 'genesis': False, 'isaacsim': False}

backend = get_backend("mujoco", model_path="robot.urdf")
```

## The `DynamicsBackend` interface

Every backend implements the same set of rigid-body dynamics primitives, so identification/calibration code never branches on which simulator is active:

- `compute_mass_matrix(q)` — `M(q)` in the equation of motion
- `compute_coriolis_matrix(q, v)` — `C(q, v̇)`
- `compute_gravity_vector(q)` — `g(q)`
- `compute_forward_kinematics(q)` — per-frame position/orientation
- `compute_jacobian(q, frame)` — stacked `[linear; angular]` geometric Jacobian
- `compute_regressor(q, v, a)` — the linear-in-parameters regressor `W` such that `τ = W(q, v̇, a) · θ`; this is the core computation identification builds on
- `compute_inverse_dynamics(q, v, a)` / `compute_forward_dynamics(q, v, τ)` — RNEA / ABA
- `nq`, `nv`, `model_format` properties

A handful of methods are optional (default to `NotImplementedError`) because not every backend can implement them efficiently: `compute_dynamics_derivatives`, `get_joint_names`, `get_frame_names`, `get_inertias`, `compute_difference`/ `compute_integrate` (Lie-group operations for free-flyer bases), `random_configuration`, and `get_model_object()` — an escape hatch that returns the underlying `pin.Model` or `mj.MjModel` for advanced features not yet abstracted (e.g. `PseudoInertia`, collision models).

See [`figaroh.backends.base.DynamicsBackend`](https://thanhndv212.github.io/figaroh-plus/api/backends/index.md) for the full signatures.

## Choosing a backend

- **Research & identification** → Pinocchio or MuJoCo
- **Contact-rich tasks** → MuJoCo or Genesis
- **Large-scale / parallel** → Genesis (GPU)
- **Sim-to-real** → Isaac Sim

## Next steps

- [Integration API](https://thanhndv212.github.io/figaroh-plus/concepts/integration/index.md) — the high-level, backend-selecting entry point (`RobotIdentificationSystem`) built on top of this interface.
- [API Reference → Backends](https://thanhndv212.github.io/figaroh-plus/api/backends/index.md) — full class docs.