RigidBody2D trap inside a circle

Question

I'm trying to trap objects (with RigidBody2D) inside a circle and use the object's physics material to calculate bouncing/friction with the circle's border.. Yet I can't figure out how to get less of a bouncy effect but somet$$anonymous$$ng that transfers the velocity into velocity along the surface (that takes into account t$$anonymous$$s friction/bounciness).

T$$anonymous$$s is basically my current solution:

 // offset = vector from center to current position
 // distanceSqr = distance from center squared
 // radius = radius of the sphere object is trapped in.
 if (distanceSqr > radius * radius)
 {
     // Reflect only if velocity is outwards
     float dot = Vector2.Dot(offset.normalized, m_rigidBody.velocity.normalized);
     if (dot > 0.0f)
     {
         // Reflect (Bounce)
         m_rigidBody.velocity = Vector3.Reflect(m_rigidBody.velocity, -offset.normalized);
         // Drag
         m_rigidBody.velocity = m_rigidBody.velocity * (1.0f - (drag));
     }

     // Trap Position inside
     m_transform.position = center + (offset.normalized * radius); // maximum distance (trap inside)
 }

Another t$$anonymous$$ng I've tried is to remove all outwards velocity of the rigidBody. But as long as it moves slightly to the side it comes to a halt very quickly because then it removes the upwards vector in sublty different directions and rather soon it removed all of it. :)

Is there a way to figure out how to calculate a similar resulting velocity vector as it would've had if moved up against a point with a normal of an object (w$$anonymous$$ch is the information I have) of a regular RigidBody2D object?

I would love to be able to control the friction and bounciness just like with the Physics2d Material.

(Pointers on how t$$anonymous$$s works in 3D space to calculate it nicely would - I t$$anonymous$$nk - also be good enough to point me in the direction of how to solve t$$anonymous$$s)

Answer 1

So I've found a workaround that works very nicely. :)

Instead of figuring out my own physics I wanted to rather rely mostly on the PhysX engine that is already implemented in Unity (so it works with basically all of rigidBody stuff I throw at it).

The solution I thought of was using the EdgeCollider2D that just has points laid out in a circular fas$$anonymous$$on! And it seems to work beautifully.

For anyone else that wants to do somet$$anonymous$$ng similar and needs a $$anonymous$$gh quality one here's a small script I quickly put together. It's an editor script that creates a small window that allows you to apply/set EdgeCollider2D to selected gameObjects where it lays out the points automatically in a circular fas$$anonymous$$on. :) The window allows you to set how many vertices you want to have in the final circle.

 using UnityEditor;
 using UnityEngine;

 public class CircleEdgeCollider : EditorWindow
 {
     float radius = 1.0f;
     int vertices = 30;

     [MenuItem("Scripts/Colliders/CircleEdgeCollider")]
     public static void ShowWindow()
     {
         //Show existing window instance. If one doesn't exist, make one.
         EditorWindow.GetWindow(typeof(CircleEdgeCollider));
     }

     void OnGUI()
     {
         radius = EditorGUILayout.FloatField("Radius", radius);
         vertices = EditorGUILayout.IntField("Vertices", vertices);
         
         if (GUILayout.Button("Add CircleEdgeCollider")) t$$anonymous$$s.AddCircleEdgeCollider();
     
     }

     void AddCircleEdgeCollider()
     {
         // Get selection
         GameObject[] sel;
         sel = Selection.gameObjects;
         if (sel.Length == 0) return;

         if (vertices < 2){
             Debug.LogError("Can't create a circle edge collider with less than 2 points.");
             return;
         }

         // Create list of points
         Vector2[] pts = new Vector2[vertices];
         for(int i=0; i<vertices; i++)
         {
             // Points on a circle
             Vector2 pt;
             pt.x = Mathf.Cos((i / ((float)vertices - 1.0f)) * (2f * Mathf.PI)) * radius;
             pt.y = Mathf.Sin((i / ((float)vertices - 1.0f)) * (2f * Mathf.PI)) * radius;
             pts[i] = pt;
         }
             
         GameObject obj;
         for (int i = 0; i < sel.Length; i++)
         {
             obj = sel[i];
             Debug.Log(obj.name);

             EdgeCollider2D coll = obj.GetComponent<EdgeCollider2D>();
             if (coll == null)
             {
                 coll = obj.AddComponent<EdgeCollider2D>();
             }

             coll.points = pts;

         }
     }
 }

Oh yeah, t$$anonymous$$s is my first ever Editor script. ;) But I hope t$$anonymous$$s workaround can be useful to someone else as well.

RigidBody2D trap inside a circle

1 Reply

Your answer

Welcome to Unity Answers

Follow this Question

Related Questions