Rigidbody wall slide without velocity loss

Question

I've some test scene set up like this, once unity enter play-mode sphere 1 & 2 will start moving forward using the same velocity. Sphere 1 will bump into some cubes along the way. The outcome I expect is both spheres will always have the same z position value, which means no speed loss during the collision for sphere 1.

Scene set up

Rigidbody of both sphere setting are : Mass = 1, Drag = 0, angular drag = 0, use gravity = false,is kinematic = false, Interpolate = Interpolate, Collision Detection = Continuous.

Physics material of both cubes and spheres are using :

Dynamic friction = 0, Static friction = 0, Bounciness = 0, Friction Combine = Minimum, Bounce Combine = Maximum.

I have been using following code to handle collision and velocity changes :

     private void OnCollisionEnter(Collision other)
     {
         Debug.Log(body.velocity);
         body.constraints = RigidbodyConstraints.FreezeRotation
                            | RigidbodyConstraints.FreezePositionY;
         body.velocity = Vector3.forward * 5f + Vector3.left * 5f;
         Debug.Log(body.velocity);
     }
 
     private void OnCollisionExit(Collision other)
     {
         body.constraints = RigidbodyConstraints.FreezeRotation
                            | RigidbodyConstraints.FreezePositionY
                            | RigidbodyConstraints.FreezePositionX;
         Debug.Log(body.velocity);
     }

According to the log message, the velocity of sphere 1 has been reduced most of the time when it collide with the cude in OnCollisionEnter(). log messages

As result, sphere 1's Z position is incremetally behind sphere 2.

Is there something I been missing out or did I missunderstood how velocity of a rigidbody work?

Answer 1

In case anyone has the same problem, I've managed to fix this with the following script :

 using UnityEngine;
 
 public class TestBoundaryCollision : MonoBehaviour
 {
     public float Speed;
     public bool Moving = false;
     private Rigidbody body;
     private SphereCollider sc;
     private RaycastHit castHit;
 
     // Use this for initialization
     void Start()
     {
         body = GetComponent<Rigidbody>();
         sc = GetComponent<SphereCollider>();
     }
 
     private void FixedUpdate()
     {
         if (!Moving)
         {
             return;
         }
 
         if (Physics.SphereCast(body.position,
                                 sc.radius + Speed * Time.fixedDeltaTime,
                                 transform.forward,
                                 out castHit,
                                 Speed * Time.fixedDeltaTime))
         {
             body.velocity = new Vector3(Mathf.Sign(castHit.normal.x) * Speed, 0f, Speed);
         }
         else
         {
             body.velocity = transform.forward * Speed;
         }
     }
 }

Answer 2

Answer by Ramlock · Mar 26, 2018 at 05:33 AM

The problem is that you're freezing the X position whenever it's not colliding, so the instant it does collide, it has no option but to come to a stop, untill your OnCollisionEnter script enables it to move again. That's why the debug shows a low Y velocity combined with a zero X velocity first, then a correct velocity on both at the end of the method (in the second debug)

Try removing the FreezePositionX altogether, and instead OnCollisionExit just make it so body.velocity = Vector3.forward * 5f;

Add comment · Show 8 · Share

x4637x · Mar 26, 2018 at 06:07 AM 0

Share

Thanks for your reply, but I do start with what you are suggesting here, the vertical velocity after the collision is half of the velocity before the collision, a result as (-2.5, 0, 2.5). And this is not what I trying to achieve.

Even though the magnitude of the resulting velocity is incorrect, which I am expecting (-5.0, 0, 5.0).

Ramlock x4637x · Mar 26, 2018 at 06:19 AM 0

Share

That is to be expected from physics regardless of friction. The problem here is conservation of momentum. You're expecting the object to generate sideways movement without spending it's forward velocity.

The best you can do from what I see is to set your velocity in a FixedUpdate ins$$anonymous$$d of OnCollision. Perhaps this will work for you:

      private void OnCollisionEnter(Collision other)
      {
          colliding = true;
          Debug.Log("CollisionEnter");
      }
  
      private void OnCollisionExit(Collision other)
      {
          colliding = false;
          Debug.Log("CollisionExit");
      }
 
     private void FixedUpdate()
     {
          if(colliding) body.velocity = Vector3.forward * 5f + Vector3.left * 5f;
          else body.velocity = Vector3.forward * 5f;
     }

x4637x Ramlock · Mar 26, 2018 at 06:37 AM 0

Share

You just give me some idea to solve this, but first I need to point it out that both OnCollisionEnter() and OnCollisionExit() are called after fixedUpdate and physic simulation.(According to this unity doc page)

So what you paste up there will still be using a slower vertical velocity for a fixedUpdate. I might need to find a way to do my own collision detection before physic simulation.

Show more comments

Rigidbody wall slide without velocity loss

2 Replies

Your answer

Welcome to Unity Answers

Follow this Question

Related Questions