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