detect side of collision in box collider 2d?

Use collider.bounds.center and compare it with the point.

 void OnCollisionEnter2D(Collision2D  collision) 
     {
         Collider2D collider = collision.collider;
  
         if(collider.name == "target")
         { 
             Vector3 contactPoint = collision.contacts[0].point;
             Vector3 center = collider.bounds.center;
 
             bool right = contactPoint.x > center.x;
             bool top = contactPoint.y > center.y;
         }
     }

This is going to be a little basic of me, but please forgive my ignorance, can anyone explain a bit as to why this works? or point me to a resource where I can read more about it. i'm a little confused.

For the curious, I solved this problem using the following code

     private string WhatSideOfTheColliderWasHit(Collider2D collision)
     {
         Vector2 PointOnBoxHit = collision.ClosestPoint(transform.position);
         Vector2 centerOfObject = collision.bounds.center;
         float xMinPoint = Mathf.Abs((collision.bounds.size.x/2) - centerOfObject.x);
         float xMaxPoint = Mathf.Abs(xMinPoint + collision.bounds.size.x);
         float yMinPoint = Mathf.Abs((collision.bounds.size.y / 2) - centerOfObject.y);
         float yMaxPoint = Mathf.Abs(yMinPoint + collision.bounds.size.y);
 
         if (PointOnBoxHit.x >= xMinPoint && PointOnBoxHit.x <= xMaxPoint && Approximately(yMaxPoint, PointOnBoxHit.y, .1f))
             return "Bottom";
         else if (PointOnBoxHit.x >= xMinPoint && PointOnBoxHit.x <= xMaxPoint && Approximately(yMinPoint, PointOnBoxHit.y, .1f))
             return "Top";
         else if (PointOnBoxHit.y >= yMinPoint && PointOnBoxHit.y <= yMaxPoint && Approximately(xMaxPoint, PointOnBoxHit.y, .1f))
             return "Right";
         else
             return "Left";
     }

The basic idea is that I grab the closest point on the collider box where the collision happened. I then get the calculate the min/max of x and y of this particular box. I then simply compare my point to these min/max values. It tells exactly what side of the box I am on.

Also Approximately is my own function that basically checks to see if the two values are within a threshold. It's like Mathf.Approximately but I can control the epsilon.

Edit: You may have a problem with certain blocks returning size = 0,0,0 in some cases. If this happens then just go after size from the component

 Vector3 size = collision.gameObject.GetComponent<BoxCollider2D>().size;

That fixed the issue for me. Then for the code above that calls for size, just replace collision.bounds.size with just size.

Here is a easier way to make that:

• Get the collision point by ClosestPoint (https://docs.unity3d.com/2019.1/Documentation/ScriptReference/Collider2D.ClosestPoint.html)

• Get the distance from collision object center (using bounds.center) to closestPoint

• Get the angle of the distance

• Draw a box somewhere and trace lines to determine wich part of the box will make a different action.

• Create a enum for each one of them.

• Check the angles and return the enum.

Here is the code:

 private static CollisionSide CheckIfFloorIsUnder(Collider2D thisCollider, Collider2D otherCollider)
     {
         var closestPoint = otherCollider.ClosestPoint(thisCollider.bounds.center);
         var distance = closestPoint - (Vector2)otherCollider.bounds.center;
         var angle = Vector2.Angle(Vector2.right, distance);
         if (angle < 135 && angle > 45)
         {
             return CollisionSide.Under;
         }
         //The rest of sides by angle
         return CollisionSide.None;
     }
 
 public enum CollisionSide
 {
     Under,
     Above,
     Sides,
     None,
 }