Simple Vs Complex Collision

In Unreal Engine 4, you have access to simple and complex collision shapes. Simple Collision are primitives like boxes, spheres, capsules, and convex hulls. Complex Collision is the trimesh of the given object. By default, Unreal Engine 4 creates both simple and complex shapes inside Physx, then, based on what the user wants (complex query vs simple query), we will use the corresponding shape.

Usage

In the Static Mesh Editor you can find the settings for Collision Complexity in the Static Mesh Settings category:

StaticMeshSettingsCollisionComplexity.png

Setting spacer.png Description
Default This will cause simple collision requests to use simple collision, and complex requests to use complex collision; the "default" behavior.
UseSimpleAsComplex This means that if a complex query is requested, the engine will still query against simple shapes; basically ignoring the trimesh. This helps save memory since we don't need to bake the trimesh and can improve performance if the collision geometry is simpler.
UseComplexAsSimple This means that if a simple query is requested, the engine will query against complex shapes; basically ignoring the simple collision. This allows us to use the trimesh for the physics simulation collision. Note that if you are using UseComplexAsSimple you cannot simulate the object, but you can use it to collide with other simulated (simple) objects.

For example: in the image below the chair on the left has simple collision, and when the Pawn above it falls onto it, it will slide off the large angled surface that covers the seat. However the chair on the right is using UseCompleAsSimple, and when the Pawn above it falls, it will land on the seat of the chair and stay there.