Portal rendering
Encyclopedia
In computer-generated imagery
and real-time 3D computer graphics
, portal rendering is an algorithm for visibility determination
. For example, consider a 3D computer game environment, which may contain many polygons, only a few of which may be visible on screen at a given time. By determining which polygons are currently not visible, and not rendering those objects, significant performance improvements can be achieved.
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 pre-compute visibility for sectors are referred to as potentially visible set
or PVS methods.
For example, in a computer game such as Descent
, the game area might be divided into several sectors. These sectors would then be connected to each other by small openings such as doors or windows. These openings are referred to 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 simplifies 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 scene, where each of these areas would be enclosed in such a polygonal body.
Portals are best suited for indoor scenes such as mazes. Outdoor scenes do not usually have door-like objects that would clearly separate one sector from another.
Computer-generated imagery
Computer-generated imagery is the application of the field of computer graphics or, more specifically, 3D computer graphics to special effects in art, video games, films, television programs, commercials, simulators and simulation generally, and printed media...
and real-time 3D computer graphics
3D computer graphics
3D computer graphics are graphics that use a three-dimensional representation of geometric data that is stored in the computer for the purposes of performing calculations and rendering 2D images...
, portal rendering is an algorithm for visibility determination
Hidden surface determination
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...
. For example, consider a 3D computer game environment, which may contain many polygons, only a few of which may be visible on screen at a given time. By determining which polygons are currently not visible, and not rendering those objects, significant performance improvements can be achieved.
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 pre-compute visibility for sectors are referred to as potentially visible set
Potentially visible set
Potentially Visible Sets are used to accelerate the rendering of 3D environments. This is a form of occlusion culling, whereby a candidate set of potentially visible polygons are pre-computed, then indexed at run-time in order to quickly obtain an estimate of the visible geometry...
or PVS methods.
For example, in a computer game such as Descent
Descent (video game)
Descent is a 3D first-person shooter video game developed by Parallax Software and released by Interplay Entertainment Corp. in 1995. The game features six degrees of freedom gameplay and garnered several expansion packs...
, the game area might be divided into several sectors. These sectors would then be connected to each other by small openings such as doors or windows. These openings are referred to 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
Clipping (computer graphics)
Any procedure which identifies that portion of a picture which is either inside or outside a picture is referred to as a clipping algorithm or clipping.The region against which an object is to be clipped is called clipping window.-Examples:...
against the portal boundaries to remove overdraw.
The use of portals simplifies the game engine
Game engine
A game engine is a system designed for the creation and development of video games. There are many game engines that are designed to work on video game consoles and personal computers...
's task of determining visible areas and objects from any given point of view of the level, and simplifies rendering by allowing it to use each portal as a viewing frustum
Viewing frustum
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. The exact shape of this region varies depending on what kind of camera lens is being simulated, but typically it is...
for the area it leads to. Ideally, portals are formed of confined areas (like doors or tunnels), connecting two complex areas of the scene, where each of these areas would be enclosed in such a polygonal body.
Portals are best suited for indoor scenes such as mazes. Outdoor scenes do not usually have door-like objects that would clearly separate one sector from another.