View source for Portal rendering

{{Short description|Technique for visibility determination in computer graphics}}

In [[computer-generated imagery]] and real-time [[3D computer graphics]], '''portal rendering''' is an algorithm for [[Hidden surface determination|visibility determination]]. For example, consider a 3D computer game environment, which may contain many [[Polygon (computer graphics)|polygons]], only a few of which may be [[Visibility (geometry)|visible]] on screen at a given time. By determining which polygons are currently not visible, and not [[Rendering (computer graphics)|rendering]] those objects, significant performance improvements can be achieved.

A portal system is based on using the [[Space partitioning|partitioning of space]] to form generalizations about the visibility of objects within those spaces. Regions of map space are divided into polygonal, generally [[Convex polygon|convex]], areas called ''zones'', or sometimes ''sectors''. Adjacent zones are linked to one another via shared dividing polygons termed ''portals''.<ref>{{cite conference |url=https://books.google.com/books?id=z0FsCQAAQBAJ&pg=PA915 |conference=Computational Science — ICCS 2003 |title=International Conference Melbourne, Australia and St. Petersburg, Russia June 2–4, 2003 Proceedings, Part I |author1=David Abramson |author2=Alexander V. Bogdanov |author3=Jack J. Dongarra |author4=Albert Y. Zomaya |author5=Yuriy E. Gorbachev |year=2003 |isbn=9783540448600 |publisher=[[Springer Berlin Heidelberg]] |page=915}}</ref> Approaches that precompute visibility for zones are referred to as [[potentially visible set]] or ''PVS'' methods.<ref>{{cite book |url=https://books.google.com/books?id=cJBWeBVLXAcC&pg=PA490 |title=Advanced 3D Game Programming with DirectX 10.0 |author=Peter Walsh |year=2010 |isbn=9781449612764 |publisher=[[Jones & Bartlett Learning]] |pages=490–511}}</ref>

For example, in a [[computer game]] such as ''[[Descent (video game)|Descent]]'', the game area might be divided into several zones. These zones would then be connected to each other by small openings such as doors or windows. These openings are referred to as ''portals''. When the zone 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 zone can be [[clipping (computer graphics)|clipped]] against the portal boundaries to remove overdraw.<ref>{{cite book |title=Tricks of the 3D Game Programming Gurus: Advanced 3D Graphics and Rasterization |author=André LaMothe |year=2003 |isbn=9780672318351 |publisher=[[Sams Publishing]] |page=1408}}</ref>

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.<ref>{{cite thesis |url=http://gmm.fsksm.utm.my/~cogramm/cgi-bin/2004/upload_files/full_paper_backup/077672.pdf |title=Dynamic Scene Occlusion Culling in Architectural Scenes |author1=Baldeve Paunoo |author2=Daut Daman |year=2004 |location=Universiti Teknologi Malaysia |degree=Master of Science (Computer Science)}}</ref>

Portals are best suited for indoor scenes such as mazes. Outdoor scenes do not usually have door-like objects that would clearly separate one zone from another.<ref>{{cite book |url=
https://books.google.com/books?id=IfDouSUqOUIC&pg=PA372 |title=Core Techniques and Algorithms in Game Programming |author=Daniel Sanchez-crespo |year=2004 |isbn=9780131020092 |publisher=[[New Riders Press]] |page=372}}</ref>

==Antiportals==
{{more citations needed|date=October 2017}}

In [[computer-generated imagery]] and real-time [[3D computer graphics]], '''antiportal rendering''' is a way to reduce overdraw (the [[Rendering (computer graphics)|rendering]] of detail which will not be in the final image), and in this way to optimize draw speed. Antiportals are the inverse of portals.

An antiportal (or occluder) works by defining a [[plane (geometry)|plane]] or [[volume]] which can ''never'' be seen through, normally by placing it within an [[Opacity (optics)|opaque]] object. The renderer uses this to quickly calculate which objects/faces/vertices lie behind the antiportal, and so are out of line of sight, so do not need to be rendered.<ref name="byrne">{{cite book |title=Game Level Design |last=Byrne |first=Edward |year=2005 |publisher=Cengage Learning |location= |isbn=1-58450-369-6 |page=[https://archive.org/details/gameleveldesign0000byrn/page/231 231] |pages= |url=https://archive.org/details/gameleveldesign0000byrn/page/231 }}</ref>

Many [[video game]]s and 3D graphics programs use this technique for speed rendering. ''[[Unreal Tournament 2004]]'', among many other games, uses this technique.<ref name="byrne" />

==See also==
* [[Binary space partitioning]]

==References==
{{reflist}}

==External links==
* [http://www.flipcode.com/archives/Building_a_3D_Portal_Engine-Issue_01_Introduction.shtml Building a 3D Portal Engine]

[[Category:Computer graphics]]
[[Category:Unreal (video game series)]]
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