Question OnCollisionEnter2D get collider position at time of contact

Continuous collision detection doesn't work with triggers as continous works by stopping at the point of impact; something which triggers don't do. Also, triggers don't produce contacts so you don't have contacts to work with.

With circle triggers overlapping, you can assume that to separate them you use the vector (line segment) between their current centers i.e. CircleA position - CircleB position. This is the direction/distance between them. The point at which they'd contact is calculated by extending this line segment either both ends or one end so that it's equal to both their radii.

Alternately, if you have velocity information then you can separate them back out along the velocity direction. You then have to project the closest point along that vector.

I just put this together for the case where they are overlapped as triggers but don't have any velocity information:

Code (CSharp):

1. using System;
2. using UnityEngine;
3.  
4. public class Separate : MonoBehaviour
5. {
6.     public CircleCollider2D CircleA;
7.     public CircleCollider2D CircleB;
8.  
9.     private void OnDrawGizmos()
10.     {
11.         if (CircleA == null || CircleB == null)
12.             return;
13.        
14.         // Fetch the circle transform details.
15.         var transformA = CircleA.transform;
16.         var transformB = CircleB.transform;
17.         Vector2 circleCentroidA = transformA.position;
18.         Vector2 circleCentroidB = transformB.position;
19.         var circleScaleA = transformA.lossyScale;
20.         var circleScaleB = transformB.lossyScale;
21.        
22.         // We need to scale the circle radius by the transform scale.
23.         var circleRadiusA = CircleA.radius * Mathf.Max(circleScaleA.x, circleScaleA.y);
24.         var circleRadiusB = CircleB.radius * Mathf.Max(circleScaleB.x, circleScaleB.y);
25.         var totalRadius = circleRadiusA + circleRadiusB;
26.  
27.         // Fetch the overlapped direction/distance.
28.         var contactDirection = circleCentroidB - circleCentroidA;
29.         var contactDistance = contactDirection.magnitude;
30.        
31.         // Edge-case where circles are exactly on top of each other.
32.         // You can choose a direction to separate or here we just early-out.
33.         if (contactDistance < Mathf.Epsilon)
34.             return;
35.        
36.         // Ignore if the circles are not overlapped.
37.         if (contactDistance > totalRadius)
38.             return;
39.        
40.         // Draw the calculated centroid.
41.         var contactNormal = contactDirection.normalized;
42.         var contactOffset = contactDistance - totalRadius;
43.         var contactCentroidA = circleCentroidA + contactNormal * contactOffset;
44.         Gizmos.DrawSphere(contactCentroidA, circleRadiusA);
45.     }
46. }
47.  

 

This isn't the callback for triggers, it's never called for them. That'll be OnTriggerEnter2D so you're getting something confused here. If you're getting OnCollisionEnter2D call then you have something that isn't a trigger.

Not for triggers.

 

Kinematic isn't affected by forces so there won't be any continuous collision detection available so it'll be discrete.

I still don't understand why my solution above won't work if you consider the velocity.

 

No, because they are no longer overlapped (apparently). You didn't mention anything about the state of them when asking the original question so I had to make the assumption (because you said they were triggers) that they had contacted and were still overlapping.

Apparently now they're not triggers and they're not overlapping so you need to look at my original reply in that context. If I had known they were not triggers and not overlapping (as shown in the image above) then it would've been a different answer.

Presumably you're using useFullKinematicContacts so yes, you can get the continuous contact information from that.

Not an error, intentional:
https://github.com/erincatto/box2d/...16230/src/dynamics/b2_contact_solver.cpp#L751

https://github.com/erincatto/box2d/...16230/include/box2d/b2_common.h#L65C12-L65C12

b2_linearSlop * -3 = 0.005 * -3 = -0.015

Physics isn't about being a geometry intersection library but instead it's about a stable rigidbody dynamics system. To achieve that, it requires various offsets/margins etc. This is one of them.

Only features that are not used by the physics system itself will give an exact intersection and that is limited to Line/Raycast which are indeed pure geometry intersection tests.