Physics2D.OverlapCircleAll not detecting other gameobjects

Question

I have a prefab of a enemy that will spawn in a random position multiple times around the player. However, sometimes this can make one enemy prefab overlap another enemy prefab.

So, I wrote a script which uses Physics2D.OverlapCircleAll() to detect any colliders before instantiating the enemy prefab which avoids the enemy prefab from overlaping an existing enemy. My issue is that the OverlapCircleAll() didn't detect the other instances of the prefab.

I already tried with Physics2D.OverlapBoxAll aswell. If I spawn more than 30 of these "enemy prefabs" , at least one will overlap another enemy

This is the code used to detect overlap:

public void SpawnEachEnemy(GameObject Enemy)
{
    Vector3 futurePosition = new Vector2(UnityEngine.Random.Range(UpperLeft.transform.position.x, DownRight.transform.position.x),
                                UnityEngine.Random.Range(UpperLeft.transform.position.y, DownRight.transform.position.y));
    bool correctPosition = false;
    while (!correctPosition)
    {
        Collider2D[] collider2Ds = Physics2D.OverlapCircleAll(futurePosition,0.2f);
        if (collider2Ds.Length > 0)
        {
            //re-spawning to prevent overlap
            futurePosition = new Vector2(UnityEngine.Random.Range(UpperLeft.transform.position.x, DownRight.transform.position.x),
                                UnityEngine.Random.Range(UpperLeft.transform.position.y, DownRight.transform.position.y));
        }
        else
        {
            correctPosition = true;
        }
    }

    GameObject b = Instantiate(Enemy) as GameObject;
    b.transform.position = futurePosition;
    b.transform.parent = this.transform;
}

Answer 1

Louis Garczynski mentioned a few of the possibilities but one that wasn't mentioned is that if these are all instantiating in the span of a single frame (a guess based on a comment saying SpawnEachEnemy is called in a loop), then you may need to enable Auto Sync Transforms under Physics2D Settings :

This minimal reproducible example when attached to the camera in a new 3D project's scene should work as you intend with Auto Sync Transforms enabled and it will fail to prevent overlaps when it is disabled. It may be what is preventing it from working for you:

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class TestScript : MonoBehaviour
{
    Vector3 upperLeft;
    Vector3 downRight;

    GameObject prefab;

    // Start is called before the first frame update
    void Start()
    {
        transform.position = new Vector3(0, 0, -3);
        upperLeft = new Vector3(-1, -1);
        downRight = new Vector3(1, 1);

        prefab = GameObject.CreatePrimitive(PrimitiveType.Sphere);
        DestroyImmediate(prefab.GetComponent<SphereCollider>());
        prefab.transform.localScale = 0.4f * Vector3.one;
        prefab.AddComponent<CircleCollider2D>();

        for (int i = 0; i < 12; i++)
        {
            SpawnEachEnemy(prefab);
        }

        prefab.SetActive(false);

    }


    public void SpawnEachEnemy(GameObject Enemy)
    {
        Vector3 futurePosition;
        Collider2D[] collider2Ds;

        do { 
            futurePosition = new Vector2(
                UnityEngine.Random.Range(
                    upperLeft.x,
                    downRight.x),

                UnityEngine.Random.Range(
                    upperLeft.y, 
                    downRight.y));

            collider2Ds = Physics2D.OverlapCircleAll(futurePosition, 0.2f)
        } 
        while (collider2Ds.Length > 0)

        GameObject b = Instantiate(Enemy) as GameObject;
        b.transform.position = futurePosition;
        b.transform.parent = this.transform;
    }
}

Answer 2

I ran into exactly this while writing some feature tests.

As addendum to Ruzihm's awesome answer (was stuck for ages until I found that.).

If your game does not explicitly need AutoSyncTransforms every frame, then its preferable to leave it off, as it can cause a performance hit.

You should only set autoSyncTransforms to true for physics backwards compatibility in existing projects

If you just need it in tests or on a loading frame then:

You can manually call a transform sync with:

Physics.SyncTransforms(); or Physics2D.SyncTransforms();

Or set Physics.autoSyncTransforms = true; at the start, and then back to false at the end of your Start() method.

Either of these is preferable as you don't incur a penalty on subsequent frames.

If your finding you must use AutoSyncTransform or SyncTransform() in normal running. Consider a Coroutine to defer instantiations so that a script isn't creating lots of things at once.

Ideally you want as many frames a second as possible, so there 'may' be little gameplay impact spawning things one/a few at a time, on sequential frames. Rather than incurring an overall performance hit and potential stuttering as a script tries to create too much at once.

Answer 3

First your code could be simplified to something like this:

public void SpawnEachEnemy(GameObject Enemy)
{
    Vector3 futurePosition;
    do
    {
        futurePosition = new Vector2(
            UnityEngine.Random.Range(UpperLeft.transform.position.x, DownRight.transform.position.x),
            UnityEngine.Random.Range(UpperLeft.transform.position.y, DownRight.transform.position.y)
        );

    } while (Physics2D.OverlapCircleAll(futurePosition,0.2f).Length > 0)

    GameObject b = Instantiate(Enemy) as GameObject;
    b.transform.position = futurePosition;
    b.transform.parent = this.transform;
}

I would also recommend adding a safety counter to your loop, to avoid an infinite loop if there is no room to be found.

Now, a lot of things could be wrong:

• Maybe the OverlapCircle and the spawn don't happen in the same place? While setting the parent won't modify the world position, I would still set the position after setting the parent. Not the issue here.

• Maybe the size of the overlap is too small? Are you sure your enemies are 0.2 units in radius? Consider using Debug.DrawLine to draw the radius of the scanned circle.

• Maybe your enemies are in a layer not in the DefaultRaycastLayers ? Try using a much larger circle radius and add a Debug.Log whenever OverlapCircleAll actually works.

• There are a few other possible reasons, like disabled colliders, or colliders that are too small, etc. This should however cover most likely mistakes.

Answer 4

Thank you, Ruzihm , I was looking for this answer for days. I had the same problem with my code using Physics2D.OverlapCircle. I am surprised I didn't find anyone who mentions Auto Sync Transforms anywhere else.

if ( Physics2D.OverlapCircle(position, radio) == null) {
   
   GameObject obstacleInst = Instantiate(obstacle, transform);

   obstacleInst.transform.position = position;
   obstacleInst.transform.localScale = new Vector3(scale, scale, 1);
   obstacleInst.transform.rotation = Quaternion.Euler(new Vector3(0, 0, Random.Range(0, 360)));
}