Bug The first call to OverlapSphereNonAlloc finds non-overlapping colliders

Hello,

I'm hitting a strange bug - I have a scene with about 1,000 spheres in it, and I'm using Physics.OverlapSphereNonAlloc to detect nearby spheres. This works as expected, except for the very first call after launch, which fills the provided array with non-overlapping colliders.

This is in Unity 2020.1.0b12.3931.

Here's some elided code to show what happens:

Code (CSharp):

1. private void Update()
2.     {
3.         Collider[] nearby = new Collider[4];
4.         int n = Physics.OverlapSphereNonAlloc(Vector3.zero, 1f, nearby);
5.         for (int i = 0; i < n; i++)
6.         {
7.             Debug.Log(
8.                 "Nearby collider " + (i + 1) + "/" + n + ": " + nearby[i].transform.name +
9.                   " at " + nearby[i].transform.position + " r=" + nearby[i].GetComponent<SphereCollider>().radius);
10.         }
11.     }

On the first call to update, this is the output:

Code (csharp):

1.  
2. Nearby collider 1/4: Sphere1 at (0.0, 0.0, 0.0) r=2.56
3. UnityEngine.Debug:Log(Object)
4. Galaxy:Update() (at Assets/FEO/Common/Scripts/Simulation/POC.cs:27)
5.  
6. Nearby collider 2/4: Sphere2 at (255.8, 5.8, 263.0) r=2.56
7. UnityEngine.Debug:Log(Object)
8. Galaxy:Update() (at Assets/FEO/Common/Scripts/Simulation/POC.cs:27)
9.  
10. Nearby collider 3/4: Sphere3 at (257.0, 6.4, 264.4) r=2.56
11. UnityEngine.Debug:Log(Object)
12. Galaxy:Update() (at Assets/FEO/Common/Scripts/Simulation/POC.cs:27)
13.  
14. Nearby collider 4/4: Sphere4 at (83.9, 6.7, 81.3) r=2.56
15. UnityEngine.Debug:Log(Object)
16. Galaxy:Update() (at Assets/FEO/Common/Scripts/Simulation/POC.cs:27)
17.  

On every subsequent frame, only 1 collider is detected.

 

Well, anyway, I just have an extra check that validates each overlap by checking the distance.

This seems buggy as hell - I'm guessing the first call to the function returns a value of `n` that's too high, but because this part of the engine is closed-source I can't really debug it without delving deeper into C# than I am willing to do.

 

The spheres are spawned in Start, and their position is set in Instantiate, but the very next line (still in Start) adjusts the position. I guess that explains it then, thanks. This was really bugging me.