Question [DOTS] How to raycast only front face

Apparently, DOTS Physics raycast can also hit the back face. I implemented a custom

ICollector<RaycastHit>

to filter out the back face by looking at the hit normal. However,

RaycastHit.SurfaceNormal

is the back face normal in case the ray hit the back face. I want to get the front face normal from raycast result, even if the ray hit the back face. How can I solve this problem?

Here is the code:

Code (CSharp):

1.   struct FrontFaceClosestHitCollector : ICollector<RaycastHit> {
2.     public bool EarlyOutOnFirstHit => false;
3.     public float MaxFraction { get; private set; }
4.     public int NumHits { get; private set; }
5.  
6.     public RaycastHit ClosestHit { get; private set; }
7.  
8.     public float3 RayDirection;
9.  
10.     public FrontFaceClosestHitCollector(float3 rayDirection) {
11.       MaxFraction = 1f;
12.       NumHits = 0;
13.       ClosestHit = default;
14.       RayDirection = rayDirection;
15.     }
16.  
17.     public bool AddHit(RaycastHit hit) {
18.       if (dot(RayDirection, hit.SurfaceNormal) > 0f) return false;
19.  
20.       MaxFraction = hit.Fraction;
21.       ClosestHit = hit;
22.       NumHits = 1;
23.       return true;
24.     }
25.   }

 

Hello Looooooooooooo and well come to this forum.

If it is a close 3d object the ray I presume will not hit the backface. And if is an open object like a leaf, I presume you can put a boolean flag toggle?

More exactly, what is the problem?

Looking the API like the idea of
https://docs.unity3d.com/ScriptReference/RaycastHit-normal.html

Code (CSharp):

1. // Draw lines to show the incoming "beam" and the reflection.
2. Debug.DrawLine(gunObj.position, hit.point, Color.red);
3. Debug.DrawRay(hit.point, reflectVec, Color.green);

 

Sometimes, I have to raycast from inside a collider. And I don't want to hit the back face of said collider. And even if I raycast from outside all the colliders, it's not guaranteed to always hit the front face first. What if the ray hit the back face of an open mesh, a plane or a quad? The thing is, DOT Physics doesn't seem to have an option to disable back face ray cast query, like in the old Physics module.

 

You're right, there's no support to disable back face hits. However, you can detect it and use the "front" normal, see this thread: https://forum.unity.com/threads/raycasthit-backface-culling.1064876/

Please let me know if this solves the problem or you need further help.

 

@milos85miki Thank you for your reference. The provided code didn't work itself at first, so I had to modify it a bit. The child polygon collider's normal needs to be transformed to world space before comparing with the raycast hit surface normal. Here is the final raycast collector code:

Code (CSharp):

1.  
2. struct FrontFaceClosestHitCollector : ICollector<RaycastHit> {
3.   // ICollector
4.   public bool EarlyOutOnFirstHit => false;
5.   public float MaxFraction { get; private set; }
6.   public int NumHits { get; private set; }
7.  
8.   // Input
9.   public NativeSlice<RigidBody> Bodies;
10.  
11.   // Output
12.   public RaycastHit ClosestHit { get; private set; }
13.  
14.   public FrontFaceClosestHitCollector(NativeSlice<RigidBody> bodies) {
15.     MaxFraction = 1f;
16.     NumHits = 0;
17.     ClosestHit = default;
18.     Bodies = bodies;
19.   }
20.  
21.   public bool AddHit(RaycastHit hit) {
22.     if (!IsFrontFaceHit(hit)) return false;
23.  
24.     MaxFraction = hit.Fraction;
25.     ClosestHit = hit;
26.     NumHits = 1;
27.     return true;
28.   }
29.  
30.   unsafe bool IsFrontFaceHit(RaycastHit hit) {
31.     var body = Bodies[hit.RigidBodyIndex];
32.  
33.     if (!body.Collider.Value.GetLeaf(hit.ColliderKey, out var childCollider)) return false;
34.  
35.     var childColliderType = childCollider.Collider->Type;
36.     var childColliderIsPolygon = childColliderType == ColliderType.Triangle || childColliderType == ColliderType.Quad;
37.  
38.     if (!childColliderIsPolygon) return false;
39.  
40.     var collider = (PolygonCollider*)childCollider.Collider;
41.     var localNormal = collider->Planes[0].Normal;
42.     var worldNormal = rotate(body.WorldFromBody, localNormal);
43.  
44.     return all(abs(hit.SurfaceNormal - worldNormal) <= EPSILON);
45.   }
46. }