AreaPortal Visibility

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AreaPortal Visibility
By Filami
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Objectives

• Practice the techniques of visibility
  determination using Area Portals
• Develop an experimental application showing
  techniques of visibility determination for
  interior scenes
• Develop Know How for future game applications in
  the style of Doom3 or Unreal Tournament.

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What are AreaPortals?

• A Technique the splits a scene in several areas
  interconnected by portals
• Lets to compute the visibility of contained
  objects fast and efficiently
• Effective on interior scenes full of rooms,
  corridors and the corresponding doors or
  portals.

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How to build a scenariowith AreaPortals?

• All scenes are built using many objects that adds
  detail and mood to the scenery
• Those scenes are not adequate for the
  construction of the data structures, they have
  got a lot of small details and is difficult to
  identity the areas.

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How to build a scenariowith AreaPortals? (cont.)?

• The scene is then separated into 2 kinds of
  objects
• A Cage that only has walls, ceilings and
  floors.
• Static Meshes with the remaining scene detail
• In the following picture, only the Cage is
  being shown it is at the same viewpoint as the
  previous picture.

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How to build a scenariowith AreaPortals? (cont.)?

• It's then inserted new models into the Cage
  that will be identified as portals that will
  connect the areas.

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DEMO
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Construction

• After the scene as been loaded from hard drive,
  this one has to be prepared
• Calculate the scene's areas
• Compute the portals and corresponding area
  connections
• But first of all, the scene has to be subdivided
  into convex volumes.

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Binary Space Partition (BSP)?

• The BSP divides the Cage into convex volumes
• A BSP is computational binary tree whose divider
  are three-dimensional planes
• The following image shows a small example of a 2D
  scene divided by a BSP.

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Binary Space Partition (Cont.)?

• It's possible to note that the BSP actually cuts
  some triangles
• Those cuts have to be minimized and that is
  possible using a subdivision heuristic that
  minimizes cuts without sacrificing the balancing
  of the tree.
• The following images shows the cuts that have
  been applied by the application.

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Binary Space Partition (Cont.)?

• The following image shows the computational tree
  generated for the actual test scene
• The blue dots are the nodes containing the
  cutting planes
• The rectangles are the tree's leaves with the
  number of Cage's faces on that leaf.

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DEMO
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Calculating the Areas

• After dividing the scene into convex volumes, it
  is then possible to determine the areas.
• The areas are computed using raytracing
• For each leaf pair, a given number of random rays
  are traced
• If one of those rays is not occluded, the two
  leaves have to be part of the same area
• The portals do not let rays pass.

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Calculating the Areas (Cont.)?
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Calculating the Areas (Cont.)?

• Apart form occluding those rays, the portals also
  uses them to determine which areas it connects.
• When a portal receives a ray coming from an area,
  the portal stores that area and the area is also
  notified that that portal is in it's inside
• The areas knows it's portals and the portals
  knows it's areas.

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Calculating the Areas (Cont.)?

• The portals have to be simplified as well
• Their geometries are simplified into rectangles
  so in the future it simplifies the visibility
  system workings.

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DEMO
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Application

• After the needed data structures had been
  computed it is then possible, through a given
  point of view, identify which the tree's leaves
  are potentially visible

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How the Portals are used?

• It is calculated a visibility frustum through the
  point of view to determine which objects are
  visible
• The portals cuts out the visibility frustum this
  way, the frustums are not able to see through the
  walls.

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DEMO
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What about Details?

• Now that the scene's areas have been identified
  and it's possible to determine which leaves are
  potentially visible, how to insert again the
  details into the original scene?

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What about Details? (Cont.)?

• Each scene's detail is represented by a polygonal
  mesh and each mesh has a Bounding Box. That box
  can be inserted, through the use of the BSP, into
  a leaf (and corresponding area)
• After being inserted into the scene, the
  polygonal mesh can then use the underlying
  structure to determine whether it is potentially
  visible to a particular point of view.

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DEMO
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Where are those techniquesbeing used?

• A lot of modern games still uses Area Portal or
  similar techniques to accelerate the drawing of
  their's interior scenes
• An example is Doom3 that used very similar
  techniques presented here both for construction
  of scenes as well for the visibility
  determination
• The Unreal Engine is also a very effective engine
  for interior scenes and it also uses some of the
  techniques presented here.

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Prey

• Prey is a 3DRealms game that uses the same game
  engine developed for Doom3
• It uses the same visibility system presented at
  the original game
• Those images where taken from the playable demo
  of the game and shows the portals on operation.

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Prey (cont.)?
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Unreal Engine

• In an article written for Game Developer Magazine
  by Lee Parry, Epic Game's lead level designer, it
  is possible to note that this engine also uses
  the concept of Cage and Detail Meshes.

Cage used for BSP and created using CSG
(Constructive Solid Geometry)?
Addition of Static Meshes giving important
details to the scene.
http//udn.epicgames.com/Three/rsrc/Three/ModularL
evelDesign/ModularLevelDesign.pdf
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Unreal Engine (cont.)?

• The following websites from UDN also explains in
  detail all the techniques used by this engine for
  scene development
• http//udn.epicgames.com/Two/IntroToUnrealEd.html
• http//udn.epicgames.com/Two/LevelOptimization.htm
  l

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Future Work...

• Create a loading and compilation tool for scenes
  to be integrated into an artistic workflow and
  level editors
• Develop CSP (Constructive Solid Geometry) to
  create Cages simplifying the artists' work
• Automating placement of portals through the scene.

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