World of Warcraft, Blizzard

CSE 381 – Advanced Game ProgrammingScene Management

Data Structures & Algorithms

Common Types of Scene Graphs• Bounding Sphere Trees

• Portals

• Binary Space Partitions

• Quadtrees

• Octrees

• Adaptive Binary Trees

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

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

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

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

Sector

Portals

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

Concave vs. Convex

• Sectors are typically Convex– Why?

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

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

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

Exporting the Mesh Example

• Done using a script

1. Search for the OCCLUDERS_MESH

2. Invert OCCLUDERS_MESH normals– i.e. make it solid

3. Subtract all PORTAL_*s from OCCLUDERS_MESH

4. Invert the normals for the resulting mesh– i.e. make it hollow

5. Export the resulting mesh, the OCCLUDERS_MESH, and the portals to a map file

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

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

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)

It's all about the splitter planes

• They separate objects

Alternative Representation

• B-Rep?

– boundary representation

BSPs help with collision stuff too

• Higher priorities for testing against stuff on the same side

Object to Plane testing

• Dictates much of BSP construction

• To Build BSPs

– test objects against splitter

– put into different groups

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

SNBSPs

• Good for sector testing

• Build them once:– select starting splitter plane– maintains nodes for each sector. Ex:

struct SnBSPNode

{

planesplitPlane;

int area; // sector ID

SnBSPNode frontNode;

SnBSPNode backNode;

}

NULL represents hollow region

Portals vs. BSPs

• Commonly used together

• Think a hybrid scene graph

• Portals:

– good for scene graph culling (to reduce rendering)

• BSPs:

– good for collision test culling

Quadtrees

• Partitions world in 2D– used for 2D and 3D games

To build a quadtree

1. Compute AABB for each object

2. Compute AABB for entire scene, put objects into this root node

a. Divide scene AABB into 4 boxes• not necessarily the same size

b. Put each object in current parent node into a box child node according to location

c. 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

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

Octrees

• An extension of quadtrees

• Work well with outdoor scenes– or scenes with large empty spaces

Octree Rendering• Frustum cull as you move down the tree

1. Make root node test node

2. If test node is leaf

– Render leaf contents and exit

3. Else if node is not in frustum

– Exit

4. Else

– For each child node:

• Set child node to test node and go to step 2.

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)?

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

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

Octrees Example

ABT Example

References

• A tutorial on Binary Space Partitioning Trees by Bruce F. Naylor, Spatial Labs, Inc.

– http://www.cs.cmu.edu/afs/andrew/scs/cs/15-463/2001/pub/www/notes/bsp_tutorial.pdf