In computer-generated imagery and real-time 3D computer graphics, portal rendering is an algorithm for visibility determination. A game level might contain many graphical polygons; only a few of which may be visible on screen at a given time. Visibility determination allows the renderer to decide which of those fall into that category and thus reduce rendering time. Computer-generated imagery (commonly abbreviated as CGI) is the application of the field of computer graphics (or more specifically, 3D computer graphics) to special effects in films, television programs, commercials, simulators and simulation generally, and printed media. ... 3D computer graphics are different from 2D computer graphics in that a three-dimensional representation of geometric data is stored in the computer for the purposes of performing calculations and rendering 2D images. ... In 3D computer graphics, hidden surface determination is the process used to determine which surfaces and parts of surfaces are not visible from a certain viewpoint. ...

A Portal System is based on using the partitioning of space to form generalizations about the visibility of objects within those spaces. Regions of map space are divided into polygonal, generally convex, areas called Sectors. Adjacent Sectors are linked to one another via shared dividing polygons termed Portals. Approaches that precompute visibility for sectors are referred to as potentially visible set or PVS methods. Potentially Visible Sets are used to accelerate the rendering of 3D environments. ...

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. A computer game is a game composed of a computer-controlled virtual universe that players interact with in order to achieve a defined goal or set of goals. ... A sector is a part of a whole. ... In rendering, clipping refers to an optimization where the computer only draws things that might be visible to the viewer. ...

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. A game engine is the core software component of a computer or video game or other interactive application with real-time graphics. ... In 3D computer graphics, the viewing frustum or view frustum is the region of space in the modeled world that may appear on the screen; it is the field of view of the notional camera. ...

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.

One of the first computer games to use portal rendering was Cybercon III.

See also

• Antiportal