Portal rendering
Splitting the world into rooms joined by "portals" in the doorways and rendering only through the portals the camera actually sees.
The scene is divided into cells (rooms) that border one another through portals — polygons in the openings between them. Rendering starts in the camera's cell and proceeds recursively through each visible portal, clipping the view frustum to the portal's outline. If a doorway falls off-screen, the room beyond it is never visited.
The method is exact and suited the dense indoor spaces of the mid-1990s: Descent (1995) built its entire geometry from linked cells, and engines such as Unreal and Prey used portals for visibility. In open outdoor scenes, where everything sees everything, it offers little.