Multiple UBX Collisions?

[Dashiva]

I'm modeling some props at the moment and I can't get collision to work right, I think it has to do with naming. I have my collision mesh made up of multiple box primitives. They are named UBX_meshname_01-UBX_meshname_07. However, when I get the model in the game no collision shows up. It seems to work right with UCX, but I was hoping to use simpler box collision. Is there a way I should be naming these to get it to work, or can there only be one UBX collision a mesh?

[JessieG]

UCX is recommended for collisions. You can still use the same box shapes. It would still be the same amount of primitives used for collision so its not worse on performance to use UCX over UBX.

I should also point out that UBX collision primitives tend to break if the mesh is non-uniformly scaled, which is why UCX is better.

[Dashiva]

That's interesting. I was working on Cryengine before this and their collision is about 5x more expensive for concave meshes than for box collision. I guess UDK just has a better algorithm.

[JessieG]

You can still use simple box primitives, just name them UCX instead. Epic says it's usually preferred to use UCX so the engine must handle them pretty well. I haven't run into any issues because of it.

[Spoof]

That's interesting. I was working on Cryengine before this and their collision is about 5x more expensive for concave meshes than for box collision. I guess UDK just has a better algorithm.

Concave or convex? Without using multiple primitives a concave mesh would require per-poly collision, which is slower in any engine.

[Dashiva]

Concave or convex? Without using multiple primitives a concave mesh would require per-poly collision, which is slower in any engine.

Convex. Sorry, I was typing drunk apparently. I had someone familiar with the CryEngine collision code tell me that one face of a convex mesh was as expensive to calculate as the four faces of a simple box collision.

[Spoof]

Convex. Sorry, I was typing drunk apparently. I had someone familiar with the CryEngine collision code tell me that one face of a convex mesh was as expensive to calculate as the four faces of a simple box collision.

Thats probably true for any engine, including UE3. Convex collision is a set of hyperplanes rather than faces, and an axis aligned box is similar but greatly simplified. However, in the grand scheme of things it really doesn't matter that much. The 'broadphase' of collision will already use sphere and box collision at the actor and component level before it gets to the mesh itself, and the performance of collision code at this level is less important than your overall approach to the design (i.e. if your world is a single mega mesh, then it's a problem!)

[Dashiva]

Another question, can collision meshes intersect? The rules seem to be different for different engines.

[JessieG]

The UDN recommends to leave an ever so slight gap between the collision primitives.