Virtual Reality - Lecture7-Networked VR

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  • 11

    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

    3

    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)

    4

    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

    5

    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

    6

    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

  • 27

    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)

    8

    Networked VR

    Some Major Bottlenecks

    Computer performance Bus bandwidth Operating system Rendering, etc. Heterogeneity of participant equipment

    Modeling

    Processing

    Networking bandwidth etc.

    NetVE servers

    9

    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

    10

    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)

    11

    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

    12

    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

  • 313

    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

    14

    Networked VR

    2. Centralized Systems

    A server is distributing everything

    Usually very limited number of players

    Latency

    Complexity

    15

    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

    16

    Networked VR

    3. Multiple Server Systems

    Multiple servers, each server is responsible for a

    subset of tasks

    17

    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

    18

    Networked VR

    4. Coordinated Multiple Servers

    Hierarchy of servers

    www.jntuworld.com

    www.jntuworld.com

  • 419

    Networked VR

    4. Coordinated Multiple Pros/Cons

    Pros

    Filtering

    Dynamic load sharing

    Can share a single world

    Cons

    Coordination is difficult

    Can increase latency

    20

    Networked VR

    Which to Choose?

    Hard choice

    Depends on task & application

    Guidelines

    Scalability: Serverless/Uncoord. multiple

    Reliability: Coordinated multiple server

    Simple: Centralized

    Interactivity: Serverless

    21

    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

    22

    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

    23

    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

    24

    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

  • 525

    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

    26

    Networked VR

    Shared Repositories Pros/Cons

    Pros

    Easy programming model

    Absolute state consistency

    Cons

    Single point of failure

    Bottleneck

    Unpredictable performance

    Communications overhead

    27

    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)

    28

    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

    29

    Networked VR

    3. Dead Reckoning

    Instead of sending frequent updates on

    objects 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

    30

    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

  • 631

    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

    32

    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

    33

    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

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