Best way to add MeshCollider on every GameObject

[ketourneau]

Hi,

We try to add mesh colliders on every GameObject in CustomInstantiator but we got some errors in unity :

public class CustomGameObjectInstantiator : GameObjectInstantiator {
  public CustomGameObjectInstantiator (Transform parent) : base (parent) { }
  
  public override void AddPrimitive (uint nodeIndex, string meshName, Mesh mesh, Material[] materials, int[] joints = null, bool first = true) {
      base.AddPrimitive (nodeIndex, meshName, mesh, materials, joints, first);
      nodes[nodeIndex].AddComponent<MeshCollider> ();
  }
}

[Physics.PhysX] cleaning the mesh failed
UnityEngine.MeshCollider:set_sharedMesh(Mesh)
CustomGameObjectInstantiator:AddPrimitive(UInt32, String, Mesh, Material[], Int32[], Boolean) (at Assets/Scripts/CustomGameObjectInstantiatorcs:12)

[Physics.PhysX] TriangleMesh::loadFromDesc: desc.isValid() failed!
UnityEngine.GameObject:AddComponent()
CustomGameObjectInstantiator:AddPrimitive(UInt32, String, Mesh, Material[], Int32[], Boolean) (at Assets/Scripts/CustomGameObjectInstantiator.cs:12)

How to avoid errors ?

[atteneder]

Hi @Blocovin ,

Thanks for reporting.

I'd have to try, but maybe it is related to the mesh flags (see this comment)

I'll try to test soon

[ketourneau]

Thank you for your answer !
I forgot to specify that we use draco, so PrimitiveDracoCreateContext.csis called and there is no MeshUpdateFlags.

[atteneder]

Thank you for your answer !
I forgot to specify that we use draco, so PrimitiveDracoCreateContext.csis called and there is no MeshUpdateFlags.

Good point!
I've created a corresponding issue. The next DracoUnity version will support this.

[ketourneau]

I fork UnityDraco and I try to set defaultMeshUpdateFlags = MeshUpdateFlags.Default and
defaultMeshUpdateFlags = MeshUpdateFlags.DontNotifyMeshUsers | MeshUpdateFlags.DontResetBoneBounds | MeshUpdateFlags.DontValidateIndices; in DracoMeshLoader.cs

I always got unity errors when I add collider on draco model at runtime

[Physics.PhysX] TriangleMesh::loadFromDesc: desc.isValid() failed! UnityEngine.GameObject:AddComponent<UnityEngine.MeshCollider> ()

[Physics.PhysX] TriangleMesh::loadFromDesc: desc.isValid() failed!

Do you have another suggestion to fix it ?

[atteneder]

Erm....no, nothing obvious. I'll try to look into it next week.

[ketourneau]

I found the solution, in Unity 2021.2.X the error is more explicit (https://issuetracker.unity3d.com/issues/physics-dot-raycast-throws-errors-when-used-on-a-navmesh).
So in Unity 2021.2.0a12 we got error : mesh must have at least three distinct vertices to be a valid collision mesh

It's a problem in our mesh, not in gltfast 😆
So I fix it with mesh.vertices.Distinct ().Count () >= 3 check.

using System.Linq;

public class CustomGameObjectInstantiator : GameObjectInstantiator {
  public CustomGameObjectInstantiator (Transform parent) : base (parent) { }
  
  public override void AddPrimitive (uint nodeIndex, string meshName, Mesh mesh, Material[] materials, int[] joints = null, bool first = true) {
      base.AddPrimitive (nodeIndex, meshName, mesh, materials, joints, first);
      if (mesh.vertices.Distinct ().Count () >= 3) {
        nodes[nodeIndex].AddComponent<MeshCollider> ();
      }
  }
}

[atteneder]

@Blocovin Great you found a solution!

My suspicion is that it isn't performant though (fetching Mesh.vertices and doing Distinct). Also LINQ increases build size drastically in my experience.

I'd try to avoid this checks in production code.

regards,
Andreas

[ketourneau]

@atteneder Indeed you are right it is not performant.
I use mesh.bounds.size to check minimal requirement, it is much more performant.

3D models are send by our customer, so unfortunately we must add this check in unity for the moment.

We will see if we can clean the mesh from our pipeline.

Thank you very much for your help and advice !