Strange Trimesh edge behavior

[AlexSilverman]

Hello,

I have a surface trimesh and a ball that rolls around on that surface, and I'm seeing the ball, when it has a low linear velocity, rolling along the interior edges, so the result is that when you would expect it to roll straight, it will instead roll as though constrained by a triangle. I saw a post in the R&D section mentioning something like this, and I was wondering a) if this was a known behavior and b) if there was a workaround.

Thanks.

- Alex

[Erwin Coumans]

a) if this was a known behavior and

Yes, most engines suffer this behaviour unless you use a workaround(Havok, Ageia, Bullet, ODE etc).

b) if there was a workaround.

Yes, the easiest is to always use the triangle normal for closest points. You can modify/set the normal of the contact point, to be the triangle normal in the callback.
See the ConcaveDemo, look for

Code: Select all

	gContactAddedCallback = CustomMaterialCombinerCallback;

bool CustomMaterialCombinerCallback(btManifoldPoint& cp,	const btCollisionObject* colObj0,int partId0,int index0,const btCollisionObject* colObj1,int partId1,int index1)
{
///overwrite the normal...
}

You could also write and register your custom sphere-triangle collision algorithm, and only return contact points using the triangle normal. See Demos/UserCollisionAlgorithm how to register a collision algorithm.

Hope this helps,
Erwin

[AlexSilverman]

Thanks for the direction. I stumbled upon that callback when looking for a way to notify objects that they have been collided with. I have a few questions about the parameters of it though.

1 - Am I correct in saying that the index for each collision object is the face number when that collision object is a triangle mesh?
2 - What does the partID represent? (This one is more curiosity)

Assuming I'm correct on #1, is there a method to obtain the normal for that face? If not, my plan is to get the information I need from the btStridingMeshInterface.

Thanks for your help.

- Alex

[Erwin Coumans]

1 - Am I correct in saying that the index for each collision object is the face number when that collision object is a triangle mesh?
2 - What does the partID represent? (This one is more curiosity)

Assuming I'm correct on #1, is there a method to obtain the normal for that face? If not, my plan is to get the information I need from the btStridingMeshInterface.

Yes, the index and partId identify the triangle, so you can use the btStridingMeshInterface to get access to the triangle.

btStridingMeshInterface allows multiple meshes, each mesh is identified by a partId. It is best to gather multiple static meshes for performance reasons (internally it builds an acceleration structure using a bvh tree, and one big tree is better then several smaller trees).

You should still consider simply copy/paste the btSphereTriangleAlgorithm and customizing it, it is probably easier.

Hope this helps,
Erwin

[AlexSilverman]

I went for the callback method because I thought it would be simpler, but I just looked at the sphere-triangle algorithm files and they do indeed look easier, and more fun

Thanks for the explanation.

- Alex