1. Lecture 3. Virtual Worlds : Representation,Creation and Simulation
고려대학교 그래픽스 연구실

2. Contents Representation of Virtual Worlds Creation of virtual Worlds
Simulation of Virtual Worlds
Simulation of Virtual Agents

3. Representation of Virtual Worlds
1. Overview
Geometry representation vs. Knowledge representation
2. Scene representation
3. Case study
OpenInventor
VRML
OpenFlight
Kitten

4. 1. Overview geometry vs. knowledge repNs
Knowledge representation in AI
First-order logic
Semantic network
Frame
Symbolic
symbolic representation
Inferencing (reasoning) by the symbolic manipulation
Expressiveness vs. effectiveness in inferencing
good for dealing with abstract concepts.

5. Object representation in CG
graphic primitives -- vertex, edge, polygon, voxel, pixel
surface representation
volumetric representation
display list
scene graph
good for representing geometrical information.

6. World representation in VR : ???
Knowledge representation in AI : Symbolic
- 1st order logic
- semantic nets
- frames
World representation in VR : ???
geometric physical
behavioral symbolic
Object representation in CG : Geometrical
- volumetric
- surface
- scene graph

7. 2. Scene representation Geometric properties Physical properties
kinematic properties
dynamic properties
Behavioral properties
mechanical
cognitive
emotional
social
Expressiveness
vs.
Effectiveness in computing
(interaction,
simulation,
rendering)

8. 3. Case study
Inventor
VRML
OpenFlight
Kitten

9. Inventor
Scene graph
A scene is represented by one (or many) scene graph(s).
scene graph = an acyclic directed graph
(nodes + action + state)
Nodes represents …
shape
attributes
camera
light

10. Scene Graphs example

11. Actions to be applied to the scene graph
rendering
picking
searching
storing
computing a bounding box

12. States are represented by the state elements…
current geometrical transformation
current material
current lighting model
current drawing style
current coordinates
current viewing specification

13. Node types shape nodes property nodes group nodes
cube, cylinder, sphere, mesh, …
property nodes
material, lighting model, texture, environments (e.g., fog)
group nodes
separator, LoD, switch

14. Other scene objects engine sensor manipulator for animation
for applying constraints between objects
sensor
a node that detects changes in the scene graph, and calls a function
data sensor
timer sensor
manipulator

15. VRML VRML : Representation of Virtual Worlds over Internet
Single format to describe ‘Everything’ about VR scene
A kind of standard format for VW representation
Platform-independent
Metaphor-independent
Network-language
Worst case consideration : Low-bandwidth, High-latency
Hyperlink(URL) to reference other objects via Internet

16. Enhanced static worlds Interaction Animation Scripting Prototyping
VRML Static 3D World
Enhanced static worlds
Interaction
Animation
Scripting
Prototyping
VRML Components
Geometrical specification
Behavioral specification
Network specification

17. Components of VRML Object representation Behavior representation
Geometrical representation
Polygon, Texture, 3D text, LoD
Parameters for behavior description
ex) DEF Headlight { field SFFloat Intensity 0.5 }
Behavior representation
Passing events between object/script
Event = EVENT_TYPE + VALUE
ex) eventIn SFBool OpenDoor
Internal / External / User event
Language-scripts to process events
function which Input is EVENT
ex) public void OpenDoor(SFBool value, SFTime ts)
various language type : Java, C, perl, Tcl/Tk, ...

18. Components of VRML (continue)
Scene representation
Geometrical relationship among objects
Description of geometrical property of objects
Hierarchical scene graph
Logical relationship between objects
Description of behavioral relationship of objects
Graph consists of links from Out-event to In-event
Network-related representation
Reference of remote object
Hyperlink to remote scene

19. DirectionalLight { # First child
Transform { # Third child - a blue box
translation
rotation
children [
Shape {
geometry Box {}
appearance Appearance
material Material { diffuseColor } # Blue } ]
] # end of children for world
#VRML Draft #2 V2.0 utf8
Transform {
children [
DirectionalLight { # First child
direction # Light illuminating the scene }
Transform { # Second child - a red sphere
translation 3 0 1
Shape {
geometry Sphere { radius 2.3 }
appearance Appearance
material Material { diffuseColor } # Red ]
[A Sample VRML file]

20. VRML file contains header -- #VRML V2.0 utf8 # -- comments
White spaces -- “,”, blank, tab, newline
Prototypes
Childeren Nodes
ROUTE statements

21. Node spec.
Collision { eventIn MFNode addChildren eventIn MFNode removeChildren exposedField MFNode children [] exposedField SFBool collide TRUE field SFVec3f bboxCenter field SFVec3f bboxSize field SFNode proxy NULL eventOut SFTime collideTime }

22. Node example Collision { children [] collide TRUE bboxCenter 0 0 0
bboxSize
proxy NULL }

23. Nodes Grouping Nodes Special Groups Common Nodes Sensors Geometry
Geometric Properties
Appreance
Interpolators
Bindable Nodes

24. Scene Graph Grouping and Children Instancing -- DEF/USE Meter, Radian
Anchor, Billboard, Collision, Group, Transform
Instancing -- DEF/USE
Meter, Radian

25. Events Routes Sensors Initilal event -> cascade
ROUTE CLICKER.enabled TO LIGHT.on
Sensors
ProximitySensor, VisibilitySensor, TouchSensor, CylinderSensor, PlaneSensor, SphereSensor
Initilal event -> cascade

26. Prototypes
Extended node types
Encapsulation
Parameterization

27. Scripts Logic into event cascade Initialize and Shutdown
Direct output(eventOut)
Field access
Browser Interface

28. Interpolators
Color, Coordinate, Normal, Orientation, Position, ScalarInterpolator
exposedField MFFloat key
exposedField MF<type> keyValue
eventIn SFFloat set_fraction
eventOut [S|M]F<type> value_changed

29. Etc. EXTERNPROTO Inline Background Fog NavigationInfo Viewpoint
WorldInfo