Bug Physics2D.OverlapCollider returning false positives

Very strange situation I've run into recently. I'm using OverlapCollider to check if I'm already overlapping with a platform's collider of a specific type. I seem to be getting some false positives. I set a breakpoint in my script to see if somehow the objects actually were overlapped and my math elsewhere was wrong. I have verified that the colliders are absolutely not overlapping. I even put this log statement in:

Debug.Log($"Bounds intersect: {raycastHit.collider.bounds.Intersects(_body.collider.bounds)}");

I've also set Physics2D.autoSyncTransforms = true;
for debug purposes, though only one of these two objects are moving, so I really don't think that's a problem.

And that prints 'false' within the condition that claims they overlap.

My question is this: is this by design? Is there meant to be some forgiveness area for two overlapping colliders? That makes sense for 'touching' colliders, but I don't think it makes sense for overlapping ones. Does anyone know if there's a filter parameter or something I can change that will prevent this from happening?

I have for the time being managed to work around it by changing the condition to raycastHit.collider.bounds.Intersects(_body.collider.bounds), but that uses the full bounding box. That works just fine in this situation, but it's really not ideal as I'd like to abstractly determine if there are any overlaps with other colliders whether they're capsule, box, circle, etc.

 

Well the reason I ask is that it's the behavior I'm seeing. I agree it would be a bad design, but I've seen plenty of bad things, poorly-named functions, etc. before

You probably won't like to hear this, but I generally do not use rigid bodies in 2D. I'm sure for good and proper physics simulations or games that are very physics-driven they're great, but for the types of projects I work on, they simply do not offer the precision, control, or responsiveness that I need. No amount of tweaking of the body types or update frequency/type or anything like that has ever removed the corner cases that I experience with rigid bodies, and working 99.8% of the time isn't good enough. To the extent that I do use them, it's for eventing purposes only (OnCollisionEnter etc.), and I always freeze position and rotation. I'm sure this is deeply irritating to a physics engineer, but I guarantee you that I am far from the only person who's needed to do this, and it's not a decision I've arrived at lightly. Even in Unity's Sample 2D projects you'll see a litany of collider casts and such designed to work around the types of problems that I deal with frequently.

I do make liberal use of the 2D Colliders though--I use them for collider casts, ray casts, etc., and I find they provide me with very reliable results that I can use to drive my movement logic off of. That's why I bring up autoSyncTransforms, because I do modify the transform directly. Generally my strategy is to set up some barriers for updating the collider transforms so that it doesn't need to happen literally every time I move an object, but I will toggle autoSyncTransforms sometimes to eliminate that as a source of an issue.

I just took some time to reproduce this in a test scene. No rigidbodies, only transform, sprite renderer, BoxCollider2D, and this script. Tested in 2021.3.9f1 and 2022.1.19f1. Project file is attached. The 'LogError' statement is hitting quite frequently for me.

Code (CSharp):

1. using System.Collections;
2. using System.Collections.Generic;
3. using UnityEngine;
4.  
5. public class FallScript : MonoBehaviour
6. {
7.     public float minV = 1;
8.     public float maxV = 5;
9.  
10.     private Vector3 _startPos;
11.     private Vector2 _velocity;
12.  
13.     private Collider2D[] _overlapBuffer = new Collider2D[5];
14.  
15.     // Start is called before the first frame update
16.     void Start()
17.     {
18.         Physics2D.autoSyncTransforms = true;
19.         _startPos = transform.position;
20.         _velocity = new Vector2(0, -Random.Range(minV, maxV));
21.     }
22.  
23.     // Update is called once per frame
24.     void Update()
25.     {
26.         int numOverlaps = GetComponent<Collider2D>().OverlapCollider(new ContactFilter2D(), _overlapBuffer);
27.  
28.         for (int i = 0; i < numOverlaps; i++)
29.         {
30.             Collider2D overlapCollider = _overlapBuffer[i];
31.             if (!overlapCollider.bounds.Intersects(GetComponent<Collider2D>().bounds))
32.             {
33.                 Debug.LogError($"At velocity {_velocity}, {gameObject.name} overlaps collider {overlapCollider.name} when bounds do not intersect. Y position: {transform.position.y}, other Y Position: {overlapCollider.transform.position.y}");
34.             }
35.             else
36.             {
37.                 transform.position = _startPos;
38.             }
39.         }
40.  
41.         transform.position += (Vector3)(_velocity * Time.deltaTime);
42.     }
43. }
44.  

Every single collider in the scene has a size y component of 1, and these are the kinds of things I'm seeing in the error logs with the above statement:

At velocity (0.00, -3.19), FallSquare (2) overlaps collider Square when bounds do not intersect. Y position: -3.304479, other Y Position: -4.32

 

Yes, I've used the Kinematic body types as well. My memory of them is that they met almost all of my requirements for update timing, but I also remember them not always adjudicating collisions in a way that worked for the specifics of my project. Would be happy to mess around with them again though to see if they can at least save me a few of the things I'm doing manually, and if what you're saying is true, there's probably just some configuration detail I was missing before. Can you see the .zip file I attached above? It has an extremely simple test scene in it.

Also just tried same script in super simple test scene modifying RigidBody2D's position rather than the transform. Bad news: same problem. Attached that version on this message too.

Code (CSharp):

1. using System.Collections;
2. using System.Collections.Generic;
3. using UnityEngine;
4.  
5. public class FallScript : MonoBehaviour
6. {
7.     public float minV = 1;
8.     public float maxV = 5;
9.  
10.     private Vector3 _startPos;
11.     private Vector2 _velocity;
12.  
13.     private Collider2D[] _overlapBuffer = new Collider2D[5];
14.  
15.     // Start is called before the first frame update
16.     void Start()
17.     {
18.         var body = GetComponent<Rigidbody2D>();
19.         _startPos = body.position;
20.         _velocity = new Vector2(0, -Random.Range(minV, maxV));
21.     }
22.  
23.     // Update is called once per frame
24.     void Update()
25.     {
26.         int numOverlaps = GetComponent<Collider2D>().OverlapCollider(new ContactFilter2D(), _overlapBuffer);
27.         var body = GetComponent<Rigidbody2D>();
28.  
29.         for (int i = 0; i < numOverlaps; i++)
30.         {
31.             Collider2D overlapCollider = _overlapBuffer[i];
32.             if (!overlapCollider.bounds.Intersects(GetComponent<Collider2D>().bounds))
33.             {
34.                 Debug.LogError($"At velocity {_velocity}, {gameObject.name} overlaps collider {overlapCollider.name} when bounds do not intersect. Y position: {body.position.y}, other Y Position: {overlapCollider.attachedRigidbody.position.y}");
35.             }
36.             else
37.             {
38.                 body.position = _startPos;
39.             }
40.         }
41.  
42.         body.MovePosition(body.position + (_velocity * Time.deltaTime));
43.     }
44. }
45.  

 

To further illustrate, with the script above, I put in a Debug.Break() call when the overlap was detected but a bounds intersection was not. Then I zoomed way in on the boxes in question and displayed their colliders. The error is about .02, not nearly small enough to be a floating point precision error. Image attached.

 

The bounds don’t really matter to me, I was just using them to test if the colliders should actually overlap. It sounds like the answer to my original question “Is this by design?” Is actually “Yes.” So OverlapCollider doesn’t test if colliders overlap, it tests if the colliders expanded by some offset overlap.

Rigidbody2d is not the substance of the question, I only brought it up because you asked why I was using autoSyncTransforms. I’ll take another look at using the kinematic ones, but as you said, has nothing to do with the issue at hand.

IsTouchingLayers is insufficient because I need to know if I’m already overlapping a collider or not on a platform that disappears and reappears, so I need to selectively ignore collision on said platform. I could potentially do this with events, but I use collider.Cast to find upcoming collisions and need to filter those collisions by known overlaps. Another part of the issue is that collider.Cast doesn't use this defaultContactOffset (at least a simple test seems to confirm that it does not).

It strikes me as wrong that an 'Overlap' function would use this contact offset because I'm specifically trying to determine if there is any space in the 'union' area of any collider with the one I'm calling the function on, and you seem to have conceded that yourself in your first message . At the very least it would be useful to be able to remove or ignore the contact offset for a call such as this.