Every 3D system needs a piece of geometry - a card, a sphere, a cube, something - to receive an image or clip that the camera can “see.” One is all you need, but you can setup complex systems with a large amount of 3D data. When you have two or more objects for a 3D system, you need a Scene node to create a “place” where the camera (and the ScanlineRender node) can see all the objects at once.