In computer-generated imagery and real-time 3D computer graphics, portal rendering is a way to reduce overdraw and in this way to optimize drawing. A portal renderer is a renderer that encloses parts of the game world ("level") in polygonal bodies. Each face of such a polygonal body can lead to another such body, thus constituting a portal.

For example, in a computer game, the game area might be divided to several sectors. These sectors would be then connected to each other by small openings such as doors or windows. These openings are referred as portals. When the sector behind a portal needs to be drawn, the only parts that are visible are the parts that can be seen through the portal. Therefore, the sector can be clipped against the portal boundaries to remove overdraw.

The use of portals simplifies the game engine's task of determining visible areas and objects from any given point of view of the level and simplifying rendering by allowing it to use each portal as a viewing frustum for the area it leads to. Ideally, portals are formed of confined areas (like doors or tunnels), connecting two complex areas of the level, where each of these areas would be enclosed in such a polygonal body.

Portals are best suitable for indoor scenes such as mazes. Outdoor scenes do not usually have door-like objects that would clearly separate one sector from another.

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