Question Can't get same result from auto and manual simulation of Physics Scene

Hi,

I've posted the question on answers.unity.com but didn't get any response, so I try here :
https://answers.unity.com/questions/1759033/physicsscenesimulate-has-different-result-than-aut.html

I am developing a tool allowing to control physics simulation.
I am duplicating the active scene and simulate the PhysicsScene manually but I can't manage to get the same result when I simulate the same PhysicsScene automatically using Physics.autoSimulation = true.

The difference is not big, it needs many assets and collisions to get something different, so it will work with very simple samples.

I have uploaded a sample project on Git.
https://bitbucket.org/redgirafegames/physicssceneerrorsample/src/master/

Here is most of the code simulating the scene :

Code (CSharp):

1. void Start()
2.      {
3.          var csp = new CreateSceneParameters(LocalPhysicsMode.Physics3D);
4.          var newScene = SceneManager.CreateScene("physics", csp);
5.          physicsScene = newScene.GetPhysicsScene();
6.          var clone = Instantiate(toClone, toClone.transform.position,
7.              toClone.transform.rotation);
8.          for (int i = 0; i < toClone.transform.childCount; i++)
9.          {
10.              var child = clone.transform.GetChild(i);
11.              var rend = child.GetComponent<Renderer>();
12.              if (rend != null)
13.              {
14.                  Debug.Log("set color");
15.                  //Call SetColor using the shader property name "_Color" and setting the color to red
16.                  rend.material.SetColor("_Color", Color.red);
17.              }
18.          }
19.          SceneManager.MoveGameObjectToScene(clone, newScene);
20.          SimulatePhysicsScene(testDuration);
21.      }
22.      private void SimulatePhysicsScene(float duration)
23.      {
24.          var t = 0.0f;
25.          while (t < duration)
26.          {
27.              physicsScene.Simulate(Time.fixedDeltaTime);
28.              t += Time.fixedDeltaTime;
29.          }
30.          Debug.Log("Physics scene simulation ended");
31.      }

Looks like a bug to me, but that's just too big I must be missing something obvious and I really don't know what...

Thanks,
Thomas

 

Some advancement :
In fact if I simulate manually the default scene, I get the same result as the Automatic simulation.

So the problem must be that the default scene and the new scene created for the physics simulation are different... but I don't see where... The scenes objects are strictly the same and I don't see any parameter or configuration when creating a new scene.

 

It's driving me nuts, Physics are just bugged...
After hours of test, I can get a different simulation results from the exact same scene before and after a Unity reboot... and it's a very simple scene with 2 spheres... Is there some random seed for Physics somewhere???

I understand that it can't be deterministic between different computers... but an extra simple scene on the same computer??

 

I have modified the sample to make it the most simple possible.
Now it is just :
- a plane
- 2 spheres
- I clone this 3 elements in a new scene by code, and set their color to red
- I manually simulate both scenes in FixedUpdate
=> I Get a different simulation result as soon as there is a collision : Please why?

Simulation start


Simulation after collision (red and black should overlap)


Here is all the code of the project :

Code (CSharp):

1. using UnityEngine;
2. using UnityEngine.SceneManagement;
3.  
4. public class GameLogic : MonoBehaviour
5. {
6.     public GameObject toClone;
7.  
8.     public Scene newScene;
9.     private PhysicsScene _physicsScene;
10.  
11.     private void Awake()
12.     {
13.         Physics.autoSimulation = false;
14.     }
15.  
16.     void Start()
17.     {
18.         var csp = new CreateSceneParameters(LocalPhysicsMode.Physics3D);
19.         newScene = SceneManager.CreateScene("physics", csp);
20.         newScene = SceneManager.GetSceneAt(1);
21.         _physicsScene = newScene.GetPhysicsScene();
22.  
23.         var clone = Instantiate(toClone);
24.         // Cloned color to differentiate
25.         for (int i = 0; i < toClone.transform.childCount; i++)
26.         {
27.             clone.transform.GetChild(i).GetComponent<Renderer>().material.SetColor("_Color", Color.red);
28.         }
29.  
30.         SceneManager.MoveGameObjectToScene(clone, newScene);
31.     }
32.  
33.     private void FixedUpdate()
34.     {
35.         _physicsScene.Simulate(Time.fixedDeltaTime);
36.         Physics.Simulate(Time.fixedDeltaTime);
37.     }
38. }

The project is still available on git to test : https://bitbucket.org/redgirafegames/physicssceneerrorsample/src/master/

Thanks

 

Is this DOTS physics? If not, standard unity physics isn’t deterministic so it will not always be the same.

 

Thanks for your reply, it's standard Unity 2019 Physics, so I don't think (I didn't know DOTS physics before your message) it's DOTS physics.

I know it's not deterministic, but I get the exact same "wrong" result if I run 10 times in a row in my editor. So I don't think that's the source of the problem, it's clearly enough deterministic (and with the simplicity of the scene... I really hope so...) to get the same result every time, but the clone scene just gives a different output.

 

Just to clarify what I say :
From what I test, the example from the documentation (simulating pool balls trajectories) is completly wrong and is absolutly not working
https://docs.unity3d.com/Manual/physics-multi-scene.html

I can't simulate the balls trajectories as I don't have the same result in 2 different scenes with the same objects and configuration.

 

Yeah because like I said, it’s not deterministic

the new Unity physics (Dots physics) is meant to be deterministic. Maybe you should investigate using that or try implementing your own physics solution that is deterministic.

 

@MelvMay Thanks ! It works for me too, that's a great starting point to investigate my bigger case!
I would have never expected that changing a game object's property during the execution of a method (inactive / active in Start execution) would have an impact... that's... disapointing...

@Antony-Blackett Thanks for your time and replies, I think @MelvMay explained way better than me what I tried to say in my previous posts.

 

That makes things way harder than I thought but I get the point, thanks for your detailed explanations.

 

Amazing !!
With the activeScene switch I really get exactly the same result even with complex scene now.
And as you say, it's really cleaner than the activate/deactivate wich was looking more like a hack.

Thank you so much! I was stuck for weeks with that.

Concerning what I want do, as I said in my first post, I'm building a tool for the asset store, that will allow to control physics simulation, so even if getting exactly the same result is not always possible, depending on very subtle things, I won't have the hand on the scene and I just want to be able to explain to my tool's users how they can get determinism or why it could fail.

Now I can, thanks again !

 

So Unity Physics (DOTS) is not deterministic across platforms? You say 'yet' so that is planned? Is that planned for only Unity Physics or classic physics as well?