At this point, the computer has no idea what in the wireframe is solid and what is void. It only knows that some lines are in front of other lines when viewed from outside the plane of the screen. For example, to the computer, the drawing of a window is only lines drawn on a plane. The computer does not know that there is an opening in the wall. The user must "build" solid objects such as walls, floors,and roofs, using the wireframe as a background, to inform the program what is solid and what is not. Where openings occur, no solids are drawn (except when the idea is to later render the object, in which case glass is drawn as a thin, solid, plane).