Unity3D Raycasting in Start behaves uncorrectly, but works correctly in Update?

Question

I have this method, which just raycasts at a position downwards, and checks what tile is there, if there is one:

    public static Tile RaycastTile(Vector3 position)
    {
        Debug.Log("raycastTile at pos: " + position);
        var layerMask = 1 << LayerMask.NameToLayer("Floor");
        RaycastHit hit;
        Debug.DrawRay(position, new Vector3(0, -10, 0), Color.red, 50);
        if (Physics.Raycast(position, new Vector3(0, -1, 0), out hit, 10, layerMask))
        {
            var tile = hit.collider.gameObject.transform;
            Debug.LogWarning(tile.name + " - " + tile.parent.name + " - at pos: " + tile.position);
            var coordinates = new Vector2Int(Mathf.RoundToInt(tile.position.x) / 2, Mathf.RoundToInt(tile.position.z) / 2);
            return map.GetTile(coordinates);
        }
        else
        {
            Debug.LogError("Tile not found under position: " + position);
            Debug.Break();
            return null;
        }
    }

Currently it is called for the player

• at Start() ("starting at: < player's absolute position >")

• and at Update() ("update at pos: < player's absolute position >")

• And the map is being generated (tiles created, destroyed, etc) in Awake().

and for some reason this is the output:

And the map generation works like this:

• Creates a room at (0, 0) with tiles and all, then it searches for a connection point on the existing map, whom the new room could be connected to. (with some rotation, if it's needed)

• (Every tile is 2x2 meters, and they are on a strict grid)

So for some reason calling this method at Start() (with the same position) hits a totally different tile. While in Update() it works correctly.

At first I thought that it's because of Destroy(), because the objects are only destroyed at the end of the frame. But I tried DestroyImmediate() and it didn't help. (And after thinking, I realised that destroying objects at the end of the frame wouldn't cause issues. Or is it?)

Then I had the idea, that maybe Start doesn't wait for Awake because it compute heavy. And the raycast in the Start hits a (yet) non translated newly generated room. So I edited the code so every room is initially created at very far away, which is relatively "infinite far away" from the player: at (9999, 9999). But the issue remained.

EDIT: With (9999, 9999) Sometimes (maybe half of the time?) now it is correctly raycasted even at start, while before it was never correct in start.

EDIT2: With (0, 0) it hits correctly at start as well, but very very rarely.

Do you have any idea what could cause this issue?

Sorry if there isn't enough info, feel free to ask. :)

Answer 1

The game world of Awake/Start/Update, and the physics world of PhysX, are actually two separate sets of data. It takes some extra work from the engine to apply changes from one world to the other.

Normally this is done once in the physics step for each FixedUpdate, but depending on frame timings, we might not get a physics step / FixedUpdate in the frame that Start runs.

My best guess here is that Start is sometimes asking the physics engine to raycast before the physics engine has been told about all of the tiles, so it gets stale data.

According to the docs, you can force a sync manually with Physics.SyncTransforms, which might help flush any pending transform updates through the pipe before you start your raycast.

Or, if your game can tolerate taking a moment to settle when the objects are first spawned, you can let Start wait for a FixedUpdate or two for the physics to be up to date before it tries to raycast against it, something like this:

IEnumerator Start() {
    // Wait until the first physics step runs...
    yield return new WaitForFixedUpdate();
    // Wait until later in that frame, 
    // after physics is stepped
    yield return null;

    // Now do physics tests.

}