Multi-Scene Physics

• Create two new Scenes (Project window > Create > Scene). Name the Scenes MainScene and AlternateScene.
• In MainScene, add a Plane object (GameObject > 3D Object > Plane). Center its position to (0, 0, 0), and change the rotation to (5, 0, 0).
• Add a Sphere object (GameObject > 3D Object > Sphere) and position it above the plane at the top end (Figure 01).

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Figure 01: MainScene with plane and sphere

• Add a Rigidbody component to the sphere (Component > Physics > Rigidbody).
• Run the Scene and observe the sphere rolling down the plane and then falling off the plane.
• In AlternateScene, set up an identical simulation, but set the rotation of the plane to (-5, 0, 0), and remove any lights and cameras from the Scene.
• Open the File > Build Settings window, and add both of your Scenes here, with MainScene being index 0 (Figure 02). The first Scene in the list is always the one Unity loads when the application starts.

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Figure 02: Two Scenes in the build

Now we have two independent Scenes, each containing a simple physics simulation.

• Create a new script file (Project window > Create > C# script) and name it PhysicsSceneLoader. Copy and paste the following code into the file.

using UnityEngine;
using UnityEngine.SceneManagement;

public class PhysicsSceneLoader : MonoBehaviour
{

     //Exposed to inspector
     public string physicsSceneName;
     public float physicsSceneTimeScale = 1;
     private PhysicsScene physicsScene;
     private void Start()
     {
          //Load the scene to place in a local physics scene.
          LoadSceneParameters param = new LoadSceneParameters(LoadSceneMode.Additive, LocalPhysicsMode.Physics3D);
          Scene scene = SceneManager.LoadScene(physicsSceneName, param);       
          //Get the scene's physics scene.
          physicsScene = scene.GetPhysicsScene();
     }

     private void FixedUpdate()
     {
          //Simulate the scene on FixedUpdate.
          if (physicsScene != null)
          {
               physicsScene.Simulate(Time.fixedDeltaTime * physicsSceneTimeScale);
          }
     }
}

• In MainScene, create a new empty GameObject and add a PhysicsSceneLoader component to it. In the component options, add the name AlternateScene in the Physics Scene Name field.
• Press Play and observe how each sphere rolls down its own plane and does not interact with any objects in the other Scene.
• Modify the value of PhysicsSceneTimeScale to adjust the simulation speed of the alternate Scene.

• In order for a physics Scene to be independent, it must be loaded with the parameter that makes it independent. It can’t be done after the Scene has been loaded or dynamically created. This is demonstrated in Start(), in the code example.
• A Scene that’s loaded this way will not auto-simulate. Because of this, you should manually call Simulate() on the physics Scene. You can get the physics Scene from the Scene object. This is demonstrated in FixedUpdate(), in the code example.
• Any raycasts or shapecasts using Physics.Raycast, etc., will not interact with objects in a local physics Scene. Instead, update your code to use the casting methods on the physicsScene object. This will also work for a Scene that was not loaded with an independent physics Scene.

//Before:
//isOnGround = Physics.Raycast(ray, out hitInfo, maxGroundDist, whatIsGround);
//After:
isOnGround = physicsScene.Raycast(ray.origin, ray.direction, out hitInfo, maxGroundDist, whatIsGround);