Download - Virtual Reality - Lecture7-Networked VR
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Networked VR
Networked
Virtual Environments
SGN-5406 Virtual Reality
Autumn 2009
NetVE
Lectures
1. Introduction to Virtual Reality, VR history
2. Human senses, Audio in VR
3. Displays for VRDemos at VTT VR Centre, Wed. 14-15
4. I/O devices, haptics
5. Tracking systems for VR
6. 3D rendering, 3D modeling, VR software
7. Networked VR, web3D
8. Augmented Reality, mobile VRDemos at Machine Engineering VR lab (K1320B), Wed. 14-15
9. Novel user interfaces (11-13)
10. Applications of VR
11. Boev: Autostereoscopic displays, description of the lab work
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Networked VR
Networked Virtual Environments
Singhal - Zyda: Networked Virtual Environments, Addison-Wesley 1999
NetVE: multi-user virtual environments which have:
Common state and space
Common presence
Common time
Communication
Avatar = 3D net person
Interaction with others and with the environment
Networked, Collaborative Networked, distributed: components of VE running on separate
machines connected by a network
Collaborative: multiple users working together
NetVE, CVE, shared VE, MUVE, DIVE, WAVE, NVE
Cyberspace (in art, literature, media)
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Networked VR
Networked Virtual Environments
A virtual environment over LAN or internet. Different environ-ments have different needs
Many applications Military simulators etc.
Tele-immersion, -conferencing
Distributed CAD, engineering, design & review
Gaming, entertainment
Online communities
Distance learning and training
Examples: Second Life
World of Warcraft
Military training centers
Game consoles with internet connection
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Networked VR
Early NetVE Systems
SIMNET (DARPA) 1983- Operation Desert Storm training
Dogfight (SGI) 1985-
DIS (DoD) Distributed Interactive Simulation
Large IEEE-standard 1993
Fewer than 300 participants
Not general enough
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Networked VR
Current Systems
HLA (DoD) High-Level Architecture, replaced DIS, IEEE standard
Using HLA, computer simulations can communicate to other
computer simulations regardless of the computing platforms
DOOM 1993-
Many academic systems
X3D & other multiuser web3D
Java-based toolkits
Computer, game console gaming
A Survey of Collaborative Virtual Environment Technologies http://www.ijvr.org/issues/issue1-2009/7.pdf
www.jntuworld.com
www.jntuworld.com
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Networked VR
NetVE Basic Components
Net-VEs are:
Distributed systems
Contend with managing network resources, data loss, network
failure and concurrency
Graphical applications
Smooth, real-time display frame rates
Interactive applications
Net-VEs consist of:
3D graphics, displays
Processing
Interaction, I/O-devices
Networking (LAN, Internet)
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Networked VR
Some Major Bottlenecks
Computer performance Bus bandwidth
Operating system
Rendering, etc.
Heterogeneity of participant equipment
Modeling
Processing
Networking bandwidth etc.
NetVE servers
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Networked VR
Distributed Environments
Shared world How to organize the transmission of updates?
Simultaneously over the network How to update changes to all?
Connecting of military simulators
Problems Compatibility
Latency
Speed of the network
Consistency
Heterogeneity
Failure management
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Networked VR
Multiuser Networked VR
NetVE
Local simulator centers, or
Remote over Internet
Very hard real-time constraints
As fair and simultaneous for everybody as possible!
Major issues and problems:
I. NetVE Basic Architectures Server organization
II. Dynamic Shared State Management How to consistently update the state of VE for all?
(Many other issues also)
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Networked VR
I. NetVE Basic Architectures
1. Serverless systems (Peer to Peer)
2. Centralized client-server systems
3. Multiple server systems
4. Coordinated multiple servers
Avoid bottlenecks Better communication models – reduce the number of
connections and messages
Better database models – distributed databases
Better decision making – make it distributed, but any given
decision is made in only one place
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Networked VR
1. Serverless Systems
Peer-to-peer players on a LAN
Each broadcasts its state directly to others
WAN: each message sent individually
Broadcast is wasteful, multicast is selective
Area-of-interest management (AOIM)
Assigns packets for multicast groups
www.jntuworld.com
www.jntuworld.com
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Networked VR
1. Serverless Pros/Cons
Pros
No central bottleneck, single point of failure
Multicast is network-efficient
Multicast subscription = filtering
Cons
Difficult to manage
Network bottleneck: O(N2 )
All broadcast packets must be examined
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Networked VR
2. Centralized Systems
A server is distributing everything
Usually very limited number of players
Latency
Complexity
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Networked VR
2. Centralized Pros/Cons
Pros
Simple
Server can filter
Cons
Server is a bottleneck
Reliability?
Latency
If total consistency: one slow, all slow
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Networked VR
3. Multiple Server Systems
Multiple servers, each server is responsible for a
subset of tasks
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Networked VR
3. Multiple Server Pros/Cons
Pros
Better reliability
Scalability: tasks distributed
by clients
by dividing the VE
Cons
Does not propagate changes
Multiple points of failure
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Networked VR
4. Coordinated Multiple Servers
Hierarchy of servers
www.jntuworld.com
www.jntuworld.com
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Networked VR
4. Coordinated Multiple Pros/Cons
Pros
Filtering
Dynamic load sharing
Can share a single world
Cons
Coordination is difficult
Can increase latency
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Networked VR
Which to Choose?
Hard choice
Depends on task & application
Guidelines
Scalability: Serverless/Uncoord. multiple
Reliability: Coordinated multiple server
Simple: Centralized
Interactivity: Serverless
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Networked VR
II. Dynamic Shared State
All participants want accurate, real-time view
Location and orientation of objects
How and when to interact
Environmental info (weather, terrain, …)
Many participants on different, remote computers
Problems:
All packets have delay (latency)
Different latency for each
Limited network bandwidth
Packets may get lost
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Networked VR
Tradeoff: Consistency - Speed
Wanna make sure everybody gets the same view?
It takes time!
Impossible to
allow change
guarantee simultaneous access
identical versions
Either lots of state changes send state updates
Or fewer guaranteed state changes send consistency messages
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Networked VR
Dynamic Shared State Management
The second basic NetVE design issue!
1. Shared repository Easy, consistent
Slow, unpredictable, overhead
2. Frequent state regeneration Continuous blind broadcast
Easy to implement
Network traffic
3. Dead reckoning Prediction
Convergence
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Networked VR
Select Matching Technique
Consistent view at all sites,
Less frequent state updates
Each site has different view,
More frequent state updates
Consistency Throughput
Shared
Repository
Frequent
State
Regeneration
Dead
Reckoning
www.jntuworld.com
www.jntuworld.com
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Networked VR
1. Shared Repositories
Common data store
Updates to the store
Reads from the store
Used in DIVE, BrickNet, etc.
For small-scale LAN systems
For high consistency systems
Shared
State
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Networked VR
Shared Repositories Pros/Cons
Pros
Easy programming model
Absolute state consistency
Cons
Single point of failure
Bottleneck
Unpredictable performance
Communications overhead
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Networked VR
2. Frequent State Regeneration
Simply send the state frequently
Typically blind network broadcasts
Multicasting and filtering reduces bandwidth
Hook into event loop or on timer
Frequent updates: fast recovery
Common, for mid-scale LAN systems
SGI Dogfight, Doom (& other games)
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Networked VR
Frequent State Pros/Cons
Pros
Simple to implement
No servers needed
Better update throughput
Cons
Considerable bandwidth
Network latency and jitter
Different update rates
No absolute consistency
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3. Dead Reckoning
Instead of sending frequent updates on
object’s position, it is calculated locally using
a last-known velocity and position
Predict from periodic updates
Converge prediction to updated position
Updates are sent less frequently
Best for large-scale WAN systems permitting
inexact state consistency
Military DIS, PARADISE, NPSNET
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Networked VR
Dead Reckoning
Prediction Linear, Quadratic, Spline
Object-specialized Currently, capable of, who is doing?
Hybrid techniques
Convergence How to correct the prediction when real state is
received?
Snap, Linear, Quadratic, Spline, Hybrids
Knowledge about behavior and computation methods at remote hosts helps!
www.jntuworld.com
www.jntuworld.com
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Networked VR
Dead Reckoning Pros/Cons
Pros
Insensitive to network latency
Low-frequency updates, reduced bandwidth
Cons
Does not guarantee identical states
More complex algorithms
Prediction model is object-specific
Prediction errors significant over poor networks
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Networked VR
Information Principle
Resources ~ M * H * B * T * P M = number of Messages H = average Hosts B = average Bandwidth T = Timeliness P = Processing cycles
Improving one may affect others. Tradeoffs
The optimal choice: application dependent
Some approaches
Packet compression: reduce B, increase P
Area-of-interest: reduce H, increase M, P, T
Level-of-detail: reduce H, B, increase M, P
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Networked VR
Challenges for NetVE
Network bandwidth
Distributed action management
HW heterogeneity, compatibility
Easy to use
Fault tolerance, failure management
Real time
Scalability (2N
) ! More players means more data 100,000+ players ?!
Realism (LOD, info filtering)
3D graphics over the Internet
www.jntuworld.com
www.jntuworld.com