Physics

Fusion 2.1 has three approaches to networking physics - Forecast, Forecast Disabled and the Physics Addon. They trade off smoothness, whether proxies can interact with physics, cost and accuracy; follow the links below for the detail on each.

Approaches

Forecast

Forecast uses extrapolation to place remote physics objects in the local time of every player, so all clients run full physics locally and reconcile against the authority. Objects are smoothly interactable from all clients. For Shared Mode this is the recommended approach for interactable networked physics. For Client-Server modes consider the tradeoffs between the precision of the Physics Addon and the reduced CPU load of Forecast.

→ Forecast Physics

Forecast Disabled

Forecast Disabled is what you get without Forecast or the addon: physics objects are synchronised from their authority to other clients the same way non-physics objects are, with proxies in remote time. By default, on proxies, their rigidbodies are set to kinematic, so all interactions are handled on the authority. It is the cheapest option and suits non-interactable physics.

→ Forecast Disabled

Physics Addon (Prediction and Resimulation)

The most accurate networking of physics is to predict and resimulate the physics the same way kinematic characters are predicted and resimulated in Fusion. For client-server games an addon provides this. Note that Unity's physics engine is not optimized for resimulation, so this approach is the most expensive.

→ Physics Addon 2.1

Choosing an approach

Another alternative is to use Quantum, which takes a different approach to game networking and has a physics engine optimized for prediction and resimulation.

Fusion 2.0

Fusion 2.0 has two approaches to networking physics. Forecast does not exist in this version.

Out of the Box

Physics runs on the authority and other clients receive the synced transform but do not run physics - the behaviour now known as Forecast Disabled. Unlike Fusion 2.1, Fusion 2.0 did not set proxy rigidbodies to kinematic by default; the linked page has the compatibility setting to match the 2.0 behaviour.

→ Forecast Disabled

Physics Addon

Full proxy prediction and re-simulation for client-server games. The Fusion 2.0 addon matches the Fusion 2.1 addon's behaviour when RunnerPhysicsSimulate2D/3D is set to SimulateAlways, Physics Authority is set to Fusion, and Physics Timing is set to FixedUpdateNetwork.

→ Physics Addon 2.0