Triangle facing direction
SketchUp is very permissive about the facing direction of your geometry. You can create "single-sided" geometry — that is, geometry with no thickness — and see it from both sides; you can even apply different materials to the different faces of each plane.
The Unreal Engine uses a different approach: it assumes that all geometry is modeled with a thickness. Any triangles that face away from the camera represent the back side of a surface, and can be assumed to be hidden from view by the front side of that surface. In order to achieve the best possible performance at runtime, it always culls those back-facing triangles to avoid spending GPU cycles rendering triangles that aren't actually visible.
We recommend always modeling your 3D objects with a thickness, so that they work as expected in Unreal. However, if you do use single-sided geometry in SketchUp, Datasmith does its best to respect your intent in the way it assigns materials and facing directions to the triangles in your Static Meshes:
- If only one side of your surface in SketchUp has a material assigned, that surface is chosen for the facing direction of the triangle in Unreal.
- Otherwise, Datasmith chooses the front side of the surface in SketchUp as the facing direction in Unreal.
- If you have a material assigned to both sides, Datasmith imports only the material from the front side.
By default, this means that triangles in single-sided geometery won't appear in your Unreal scene when you look at them from the back side. In addition, the back-facing side does not cast shadows.