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Unity Engine

Manually set physics simulation

Control over when physics calculations occur to align them with game performance.
Read time 2 minutesLast updated 13 days ago
Control the physics simulation manually to decide when physics calculations occur to align physics updates with your application's actual performance and avoid redundant updates. This can help optimize performance during busy frames and is beneficial for scenarios that require tight synchronization with game logic or custom physics update loops.
Manually control the physics simulation to:
  • Ensure stability and consistency: Physics systems are designed for small, consistent time steps. Variable time step values, such as 
    Time.deltaTime
    , can cause unstable simulations, and lead to issues such as objects passing through each other, a phenomenon known as tunneling, at low frame rates or jittery behavior when frame rates fluctuate.
  • Create reproducible behavior: Using a fixed step is crucial for more reproducible physics behavior (though true determinism with PhysX has other challenges).
  • Avoid large, inaccurate steps: If frame rates drop, 
    Time.deltaTime
     increases. Simulating a physics step with a large, variable delta can be computationally resource intensive and less accurate, potentially worsening performance issues.
Use 
Physics.Simulate(float stepTime)
(or 
yourPhysicsScene.Simulate(float stepTime)
) to manually control the physics simulation and in specific, local 
PhysicsScene
instances. Refer to the Multi-Scene Physics learn tutorial to learn more about multi-scene physics.
When you call 
Physics.Simulate
, the recommended best practice is to use a fixed time step, rather than directly passing 
Time.deltaTime
. For example, use the same value that you might use in the
Project Settings

?

'
Fixed Timestep

?

value, such as 0.02 for 50Hz.
If you manually simulate physics in a scene using 
Physics.Simulate()
, ensure that automatic simulation is disabled. You can do this by setting the Simulation Mode to Script. Once in Script mode (either through Project Settings or script), you must call 
Physics.Simulate
for the scene at a selected frequency with an appropriate, preferably fixed, time step.
To disable automatic simulation, in the Editor:
  1. Select Edit > Project Settings to open the Project Settings window.
  2. Select the Physics > Settings tab.
  3. Select the
    GameObject

    ?

    tab.
  4. Set Simulation Mode to Script.
To disable automatic simulation, in script, set 
Physics.simulationMode = SimulationMode.Script;
. Typically, you do this at the start of your game, before any physics simulation occurs.

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