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Question by tfishell · Dec 22, 2020 at 02:46 AM · c#unity 2drigidbody2dphysics2dcasting

How to boxcast where a dynamic rigidbody2D's box collider will be in the next frame?

Essentially I'm trying to boxcast (Physics2D.BoxCastAll, in FixedUpdate) a dynamic rigidbody2D's box collider one frame ahead of where the rigidbody2D will end up (to see what it will collide with in the next frame, if the boxcast hits something). I'm using the velocity, scaled down by * Time.fixedDeltaTime, but currently the boxcast is much farther out than I would expect, a distance/magnitude too big.

Here's a crude drawing of what seems to be happening.

alt text

     private Rigidbody2D rb2d;
     private BoxCollider2D bc;
     private RaycastHit2D[] cast;
 
     // Use this for initialization
     void Start () {
         rb2d = GetComponent<Rigidbody2D>();
         bc = GetComponent<BoxCollider2D>();
         cast = new RaycastHit2D[5];
     }
     
     // Update is called once per frame
     void FixedUpdate () {
         
         if (!rb2d.isKinematic)
         {
             cast = Physics2D.BoxCastAll(rb2d.position, bc.size, rb2d.rotation, rb2d.velocity * Time.fixedDeltaTime);
 
             //Debug.Log(rb2d.position + ", " + (rb2d.position + (rb2d.velocity * Time.fixedDeltaTime)));
             //Debug.DrawRay(rb2d.position, Vector2.up, Color.white); //rb2d.position + (rb2d.velocity * Time.fixedDeltaTime)
 
             //Debug.DrawRay(rb2d.position + (rb2d.velocity * Time.fixedDeltaTime), rb2d.velocity * Time.fixedDeltaTime, Color.magenta);
 
             if (cast.Length > 0)
             {
                 foreach (RaycastHit2D tran in cast)
                 {
                     Debug.Log(tran.collider.gameObject);
                 }
             }
             
         }
     }

Thanks for any help.

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avatar image xxmariofer · Dec 22, 2020 at 08:35 AM 0
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whats the

 rb2d.velocity * Time.fixedDeltaTime

for? the box gets casted in

 rb2d.position
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Answer by xxmariofer · Dec 23, 2020 at 08:40 AM

Ok; I think i have just understood your issue, it was driving me crazy because to my understanding a direction made no sense there, we were confusing Physics2D.BoxCast, that its similar to Physics.RayCast but with a "bigger" ray with Physics2D.OverlapBox, here is the documentation

https://docs.unity3d.com/ScriptReference/Physics2D.OverlapBox.html

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