Advanced

1
Advanced Scene Management
2
Advanced Scene Graphs

• This is a game-type-oriented issue
• Bounding Volume Hierarchies (BVHs)
• Binary space partition trees (BSP Trees)
• Quake
• Octree
• PVS (Potentially visible set)
• Culling Skills

3
Bounding Volume Hierarchies (BVHs)

• Hierarchical structure of bounding spheres

R
B
4
BSP Tree

• Two varients
• Axis-aligned
• Polygon-aligned
• The trees are created by using a plane to divide
  the space into two, and then sorting the geometry
  into two spaces.

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Axis-aligned BSP Tree
0
plane0
2
plane3
1
plane2
plane1
3
6
Polygon-aligned BSP Tree
A
F
C
G
B
A
B
C
D
E
D
F
G
E
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Why BSP Tree ?

• Quickly to identify where you are
• BSP Sorting
• Need a pre-processor to generate the PVS
• Visibility culling occlusion culling
• PVS Potentially Visible Set
• Optimized for in-door game environment
• Fuch80
• Fuchs, H.,
• On Visible Surface Generation by a Priori Tree
  Structures,
• Computer Graphics, 14, 124-33, (Proc.
  SIGGRAPH80)

8
Octree Quadtree

• Octree
• Similar to axis-aligned BSP tree
• A box is split simultaneously along all three
  axes
• The split point must be the center of the box
• This creates eight new smaller boxes
• Quadtree is the 2D version of octree

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Quadtree - Example
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Octree Some Discussion

• Data structure coherence
• Apply visibility culling from parents
• Split or not split ?
• Outdoor game scene ?

11
Culling (1/2)

• Culling means remove from a flock
• Visibility culling
• Remove the object not in view frustum
• A must for game engine
• Backface culling
• Remove the polygons facing away from camera
• Hardware standard
• Occlusion culling
• Remove the objects hidden by the others

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Culling (2/2)
View frustum
Occlusion culling
eye
Visibility culling
Backface culling
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BSP Implementation

• A Pre-processor
• Space partition the scene data from artist
• Generate the BSP data structure
• Generate the PVS
• BSP Walk Through
• Identify the room where you are
• Show/hide the rooms according to the PVS

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BSP Preprocessor (1/2)

• Input
• A scene from artist
• Cutting planes (optional)
• Can be procedurally generated by algorithm
• Cutting policy
• Split or not split
• Ray casting resolution for PVS
• Output
• A BSP file
• BSP Tree
• PVS
• Geometry Data

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BSP Preprocessor (2/2)

• Process
• Generate the BSP tree according to the cutting
  policy
• Split or sort the geometry into BSP room (leaves)
• For each room, perform ray casting for all
  rooms to generate the possible visible room set
• 3D
• Time consuming
• Pre-processing process (can be done off-line)

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BSP Challenges

• Effectiveness of PVS
• Data set
• Dynamic Objects
• Room size