UDK and deleting faces

Benjam

Hey guys,

I was just wondering when you are making something in maya or max is it allowable to delete faces that wont be seen by the player like side of walls, undersides etc. Or should they always be filled in?

The attachment shows what i mean

Thanks guys!

Eric Chadwick

Yes they can be open. There are some caveats for special things, like if you are making a collision shape the mesh must be closed, and Fracture and FogVolumes both need a closed mesh.

Benjam

Oh sweet then for the purpose of just making an environment for the time being i can keep the faces open but for biger project where i need collision i have to completely close the model. Why is this?

Eric Chadwick

No, the render model should have unseen faces removed. But the simpler collision-only mesh should be watertight. Not sure why exactly, might be an optimization thing? Or it might be they mean "convex" instead...
http://udn.epicgames.com/Three/CollisionTechnicalGuide.html#Collision%20Types

WarrenM

Benjam

He means the collision primitives that you create for the mesh have to be closed. The mesh itself doesn't UNLESS you're colliding directly with the mesh itself. In that case, logically, no polygon means no collision.

Having open faces is usually bad for one reason - shadow casting. Lights behind the mesh will shine through it in weird ways since there's nothing to block them. You can get around this by telling meshes to cast lighting as if their polygons were 2 sided but that can be flaky at times.

Benjam

Ok thats cool, i'm on the right lines now about collision and stuff but for a general rule should i keep every mesh i create in maya closed for shadow purposes?

WarrenM

It's easiest, yes.

If you know the player will never see those polys in the back, you can UV them to a tiny corner of the diffuse/lightmap and they won't really cost you anything in terms of texture res or lighting. They'll just be there to allow for proper shadow casting.

Benjam

wicked, gotcha thanks for the advice i will post some stuff up once i have my pieces put together