1. CSE 381 – Advanced Game Programming Scene Management
Data Structures & Algorithms
World of Warcraft, Blizzard

2. Common Types of Scene Graphs
Bounding Sphere Trees
Portals
Binary Space Partitions
Quadtrees
Octrees
Adaptive Binary Trees

3. Bounding Sphere Trees Scene graph & frustum culling
Subdivides world hierarchically
Uses spherical regions
Leaves of tree contain bounding sphere of model
Models grouped in parent spheres
Groups grouped into larger spheres
Root sphere has entire scene

4. Bounding Sphere Tree Culling
1) Make root node the current node
2) If frustum collides with current node sphere:
a) if current node is a leaf, put in display list
b) else send each child node to Step 2) as current node

5. Spatial Partitioning Bonus
Reduces what's sent to rendering system
Reduces collision tests
Only test models against models in the same group
leaf vs. leaf

6. Portals First, world/map is divided into Sectors
i.e. areas/zones
Sectors are connected via Portals
typically placed manually by level designers
Sector
Sector
Portals
Sector

7. Players and Portals Players move about level
When player is in a Sector:
send items in that sector for frustum culling
Players move between sectors
Players may see from one sector into another
Sector
Sector

8. Concave vs. Convex
Sectors are typically Convex
Why?

9. Designer Strategy Sectors are not typically uniformly shaped
But must be closed geometry
No intersecting faces
No holes
How can the game designer specify sectors?
place portals
placed as very thin box
add occluders to level:
walls, fences, buildings, (cannot be planes)
separate and hide objects in sector from rest of level
automatically calculate sectors

10. Solid vs. Hollow Objects
Models are solid objects
normals face out
Rooms (interiors) are hollow objects
normals face in
may not be viewable from outside
Combinations of these are used to build levels

11. Occluders Mesh Laid out by level designer
Joins all occluder meshes in one mesh. Ex:
in modeler, combine and rename OCCLUDER_MESH
specify all portals as PORTAL_...
build and export the level mesh
A low-res mesh
simple wall geometry
typically less than 200 triangles so cheap rendering

12. Exporting the Mesh Example
Done using a script
Search for the OCCLUDERS_MESH
Invert OCCLUDERS_MESH normals
i.e. make it solid
Subtract all PORTAL_*s from OCCLUDERS_MESH
Invert the normals for the resulting mesh
i.e. make it hollow
Export the resulting mesh, the OCCLUDERS_MESH, and the portals to a map file

13. Sector Generation Remember, this must be a closed geometry
Walk through the vertices:
visit all those in each closed geometry
mark each group in common sector

14. What do you use it for?
Sector testing. Ex: find the sector a point lies in
Kind of like frustum culling
Start at the root node
Walk down to find sector
Scene Graph Culling. Ex:
render Occluder mesh (cheap)
test portals against frustum
render sectors connected to portals that pass

15. BSP Trees Binary Space Partition
to determine what sector an object is in
Each level uses splitter plane to divide into half spaces
convex half spaces
Three Types:
regular, leafy, and solid node (SNBSP)

16. It's all about the splitter planes
They separate objects

17. Alternative Representation
B-Rep?
boundary representation

18. BSPs help with collision stuff too
Higher priorities for testing against stuff on the same side

19. Object to Plane testing
Dictates much of BSP construction
To Build BSPs
test objects against splitter
put into different groups

20. Painter's Algorithm Uses regular BSP type
Separates polygons by half-spaces
Why?
polygon-to-polygon occlusion testing
We don't need this. Why?
hardware does this anyway

21. NULL represents hollow region
SNBSPs
Good for sector testing
Build them once:
select starting splitter plane
maintains nodes for each sector. Ex:
struct SnBSPNode {
plane splitPlane;
int area; // sector ID
SnBSPNode frontNode;
SnBSPNode backNode; }
NULL represents hollow region

22. Portals vs. BSPs Commonly used together Think a hybrid scene graph
good for scene graph culling (to reduce rendering)
BSPs:
good for collision test culling

23. Quadtrees
Partitions world in 2D
used for 2D and 3D games

24. To build a quadtree Compute AABB for each object
Compute AABB for entire scene, put objects into this root node
Divide scene AABB into 4 boxes
not necessarily the same size
Put each object in current parent node into a box child node according to location
Repeat a-b until decomposition limit reached
Note: all objects end up in leaf nodes
may choose to limit at most 2 objects per leaf

25. To use a quadtree Walk down the tree to test points
for placing objects
What’s this good for?
What quads are currently in frustum?
Scene graph culling
How far are quads from camera
LOD for terrain

26. Octrees An extension of quadtrees Work well with outdoor scenes
or scenes with large empty spaces
px-Octree2.svg.png 26

27. Octree Rendering Frustum cull as you move down the tree
Make root node test node
If test node is leaf
Render leaf contents and exit
Else if node is not in frustum
Exit
Else
For each child node:
Set child node to test node and go to step 2.

28. Node Confusion With Quad/Octrees, what if an object overlaps a box?
split the geometry and put in two nodes?
store object in common parent node?
loose octree
use Adaptive Binary Trees (ABT)?

29. ABT Nodes can overlap Nodes can change
Good at managing dynamic geometry at runtime
By the way, there are many other custom techniques used as well

30. So which approach is best?
There is no “best”?
Modern game engines usually use hybrid
Ex: Ogre3D
Use BSPs for closed areas
Use Octrees for open areas

31. Octrees Example

32. ABT Example

33. References
A tutorial on Binary Space Partitioning Trees by Bruce F. Naylor, Spatial Labs, Inc.
463/2001/pub/www/notes/bsp_tutorial.pdf