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Networked VR

Networked

Virtual Environments

SGN-5406 Virtual Reality

Autumn 2009

ismo.rakkolainen@tut.fi

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

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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

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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

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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

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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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Networked VR

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!

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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

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