User’s focus is typically nearby User’s focus is typically nearby

(1)

Exploiting Level

Exploiting Level- -of of- -Detail Perception Detail Perception

Nearby viewers Nearby viewers

expect full graphical expect full graphical detailsdetails

accurate structure, position, orientationaccurate structure, position, orientation

update rate update rate →→local frame ratelocal frame rate

Distant viewers Distant viewers

can tolerate can tolerate lesslessgraphical graphical detailsdetails

less accurate structure, position, orientationless accurate structure, position, orientation

User’s focus is typically nearby User’s focus is typically nearby

Many inaccuracies cannot even be detected on a fine Many inaccuracies cannot even be detected on a fine- - resolution display

resolution display

A

Multiple

Multiple- -Channel Architecture Channel Architecture

Multiple independent data channels for each entity Multiple independent data channels for each entity

Low-Low-resolution channelresolution channel (

(xx, , yy)) ((xx, , yy)) High

High--resolution channelresolution channel

High High--frequency,frequency, highhigh--bandwidthbandwidth information information Low Low--frequencyfrequency,, low low--bandwidthbandwidth information information

⇒The overall bandwidth ⇒

The overall bandwidth requirements are reduced requirements are reduced

Implementation Examples Implementation Examples

Client Client- -server server

each transmission identifies its channeleach transmission identifies its channel

server dispatches data from channels to clientsserver dispatches data from channels to clients

Multicast group for each region Multicast group for each region

assign multiple assign multiple addressesaddressesfor each regionfor each region

~~one group provides all of the entities’ highone group provides all of the entities’ high--resolution channels,resolution channels, another group provides all of the entities’ low

another group provides all of the entities’ low--resolution channelsresolution channels

Multicast group for each entity Multicast group for each entity

assign multiple assign multiple addressesaddressesfor each entityfor each entity

Different reliabilities to each channel Different reliabilities to each channel

lowlow--frequency updates are frequency updates are importantimportant

~~lost packets can have a significant impactlost packets can have a significant impact

Selecting the Channels to Provide Selecting the Channels to Provide

How many channels to provide for an entity? How many channels to provide for an entity?

more channels: better service for subscribers more channels: better service for subscribers

each channel imposes a cost (bandwidth and each channel imposes a cost (bandwidth and computational)

computational)

To satisfy the To satisfy the trade trade- -off off, three channels for each entity , three channels for each entity is typically needed

is typically needed

channels channels provide order provide order- -of of- -magnitude differences in magnitude differences in

~~structural and positional accuracystructural and positional accuracy

~~packet ratepacket rate Rigid

Rigid--body channelbody channel Approximate

Approximate--body channelbody channel Full

Full--body channelbody channel Far

Far--range viewersrange viewers Mid

Mid--range viewersrange viewers Near

Near--range viewersrange viewers

Rigid

Rigid- -Body Channel Body Channel

Demands the least bandwidth and computation Demands the least bandwidth and computation

Represents the entity as a rigid body Represents the entity as a rigid body

Ignores changes in the entity’s structure Ignores changes in the entity’s structure

Update types: Update types:

positionposition

orientationorientation

structurestructure

Approximate

Approximate- -Body Channel Body Channel

More frequent position and orientation updates More frequent position and orientation updates

Hosts can render a rough approximation of the entity’s Hosts can render a rough approximation of the entity’s dynamic structure

dynamic structure

appendages and other articulated partsappendages and other articulated parts

Provided information is entity Provided information is entity- -specific specific

correspondscorrespondsto the dominant changes of the structureto the dominant changes of the structure

(2)

Common Approximations Common Approximations

Radial lengthRadial length

motion towards and away from a motion towards and away from a centre point

centre point

update packets include the update packets include the current radius current radius

Articulation vectorArticulation vector

the current direction of the the current direction of the appendage

appendage

models a rotating turret, arms and models a rotating turret, arms and legs

legs

Local coLocal co--ordinate system pointsordinate system points

subset of the entity’s significant subset of the entity’s significant vertices relative to the entity’s vertices relative to the entity’s local co

local co--ordinate systemordinate system

the entity is composed of the entity is composed of multiple components multiple components

Radius Radius

Full- Full -Body Channel Body Channel

Highest level of detail Highest level of detail

High bandwidth and computational requirements High bandwidth and computational requirements

viewerviewercan subscribe to a limited number of full-can subscribe to a limited number of full-body body channelschannels

Frequent transmissions Frequent transmissions

Position and orientation Position and orientation

Accurate structure information Accurate structure information

Exploiting Temporal Perception Exploiting Temporal Perception

Render the entity in an accurate location Render the entity in an accurate location — — albeit slightly out- albeit slightly out - of of- -date date

As long as the local user does not interact, small temporal As long as the local user does not interact, small temporal inaccuracies

inaccuracies can be can be allowed allowed

Advantages Advantages: :

worksworkson on WANs WANs having great latencyhaving great latency

cancanenhance packet aggregationenhance packet aggregation

cancanenhance dead reckoningenhance dead reckoning

Active and Passive Entities Active and Passive Entities

An active entityAn active entity

takes actions on its owntakes actions on its own

generates updatesgenerates updates

human participants, computerhuman participants, computer-- controlled entities controlled entities

cannot be predicted typicallycannot be predicted typically

rendered using state updates rendered using state updates adjusted for the latency adjusted for the latency

A passive entityA passive entity

reacts to events from the reacts to events from the environment, does not generate environment, does not generate its own actions

its own actions

inanimate objects (e.g., rocks, inanimate objects (e.g., rocks, balls, books)

balls, books)

active entities interact with active entities interact with passive entities passive entities

rendered according to the latency rendered according to the latency of its nearest active entity of its nearest active entity

reactsreactsinstantaneously to instantaneously to the the actions

actionsof of a nearbya nearbyactive entityactive entity

Example: Pong Example: Pong

Two active entities: Two active entities:

paddles paddles

movement unpredictable movement unpredictable

One passive entity: ball One passive entity: ball

movement predictable movement predictable

Latency of Latency of d d seconds seconds d d

View of

View of the Blue the Blue Player Player

(3)

View

View of the of the Red Player Red Player Pong: Pong: A Summary A Summary

Each player sees a different representation of Each player sees a different representation of the same the same playing playing field

field

The ball accelerates as it approaches the local player’s paddle The ball accelerates as it approaches the local player’s paddle

The ball decelerates as it approaches the remote player’s The ball decelerates as it approaches the remote player’s paddle

paddle

The ball’s rendered position alternates between The ball’s rendered position alternates between

the current timethe current time

~~meaningful interaction for local playermeaningful interaction for local player

a past time referencea past time reference

~

~network latencynetwork latency

~~observing meaningful interaction for remote observing meaningful interaction for remote playerplayer

3½ 3 ½- -Dimensional Playing Field Dimensional Playing Field

Represent each player’s perception as a four Represent each player’s perception as a four- - dimensional

dimensional co co- -ordinate ordinate system ( system (x

x,

, y

y,

, z

z,

, t

t)

)

xx, , yy, , zz: the spatial position relative to the local : the spatial position relative to the local player’s current position

player’s current position

~~local player at (local player at (0, 0, 00, 0, 0))

tt: the time associated with rendered information : the time associated with rendered information from that position

from that position

~~local player rendered at current time: local player rendered at current time: t t = 0= 0

~

~opposing player: opposing player: t t = −= −dd

(0, 0, 0 (0, 0, 0))

d d

Co- Co -ordinate ordinate Systems Systems

t t = 0= 0

t = −t = −dd

Blue Player Blue Player

t t = −= −dd

t t = 0= 0

Red Player Red Player

Properties of the Co

Properties of the Co- -ordinate System ordinate System

The coThe co--ordinate system is defined ordinate system is defined independently for each player independently for each player

Depends on the player’s current Depends on the player’s current position and the delay of arriving position and the delay of arriving information

information

Changes dynamically as the player Changes dynamically as the player moves or as the network properties moves or as the network properties change

change

Defines how a passive object Defines how a passive object should be rendered should be rendered

Each user perceives all collisions Each user perceives all collisions correctly

correctly

Objects that approach the local Objects that approach the local user are rendered in the user’s user are rendered in the user’s timetime

Smooth movementSmooth movement

Generalizing the Local Temporal Contour Generalizing the Local Temporal Contour

Limitations: Limitations:

players are capable of moving along a single axis onlyplayers are capable of moving along a single axis only

supports twosupports twoactive objects onlyactive objects only

Generalization to a 4D Generalization to a 4D co co- -ordinate ordinate system system requires preserving requires preserving for the local user:

for the local user:

interactinginteractingnaturally withnaturally withpassive objects passive objects in vicinityin vicinity

seeingseeingremote interactions remote interactions (passive(passive--toto--passive,passive,passivepassive--toto--active)active)

(4)

Local Temporal Contour Local Temporal Contour

The local user at ( The local user at (0, 0, 0 0, 0, 0) )

Each active object is Each active object is assigned a assigned a t

t

value value corresponding

corresponding to its latency to its latency

Interpolate Interpolate the contour the contour over over all active objects including all active objects including local

local

Contour defines a suitable Contour defines a suitable t

t

value for each spatial point value for each spatial point

local local

t t y y

x x

Limitations Limitations

Varying latency can cause entities to (unnaturally) jump Varying latency can cause entities to (unnaturally) jump forward or backward in time

forward or backward in time

use use averagedaveragedlatency to dampen the effectlatency to dampen the effect

What if an update What if an update packet is packet is delayed

delayed considerably? considerably?

predict entity’s past position, dead reckoningpredict entity’s past position, dead reckoning

Computational requirements Computational requirements

compute compute the contourthe contourusing only the nearest active entitiesusing only the nearest active entities

§6.4 Enhancing the System Architecture

Change the network software architecture Change the network software architecture

Basic structures: client Basic structures: client- -server and peer server and peer- -to to- -peer peer

Augment and combine basic structures Augment and combine basic structures

server clustersserver clusters

~~partition clients across multiple serverspartition clients across multiple servers

~

~partition the NVE across multiple serverspartition the NVE across multiple servers

server hierarchiesserver hierarchies

peerpeer--server systemsserver systems

M

M HH BB TT PP

Traditional Client

Traditional Client- -Server Server

Server may act as Server may act as

broadcast reflectorbroadcast reflector

filtering reflectorfiltering reflector

packet aggregation serverpacket aggregation server

Scalability problems Scalability problems

all traffic all traffic goes throughgoes throughthe the server

server

⇒⇒Server clusters

Server clusters

SS CC

CC CC C C

C C C

C CC

C C C C

CC C C CC C

C

C C

Partitioning Clients

Partitioning Clients across across Multiple Servers Multiple Servers

The servers exchange control The servers exchange control messages among themselves messages among themselves

inform the interestsinform the interestsof their of their clients

clients

Reduces the workload on each Reduces the workload on each server

server

Incurs a greater Incurs a greater latency latency

The total The total processing and processing and bandwidth requirements are bandwidth requirements are greater

greater

S S S

S SS

S S C

C

C C C C CC

C C C

C CC

C C C C

C C

C C C C C

C

CC

Partitioning the NVE

Partitioning the NVE across across Multiple Servers Multiple Servers

Each server manages clients Each server manages clients located within a certain region located within a certain region

Client communicates with Client communicates with different serves as it moves different serves as it moves

Eliminates a lot of network trafficEliminates a lot of network traffic

Requires Requires advancedadvancedconfigurationconfiguration

Is Is a regiona regionvisible from another visible from another region?

region?

AggregationAggregationservers are servers are a speciala special case of NVE server partitioning case of NVE server partitioning CC CC

CC C C

CC S

S SS

S

S SS

(5)

Server Hierarchies Server Hierarchies

Servers themselves Servers themselves act asact asclientsclients

Packet fromPacket froman upstreaman upstreamserver:server:

deliver to deliver to the interestedthe interested downstream clients downstream clients

Packet from Packet from a downstreama downstreamclient:client:

deliver to deliver to the interestedthe interested downstream clients downstream clients

if other regions are interested in if other regions are interested in the

the packetpacketthen then deliver itdeliver itto the to the upstream

upstreamserverserver CC CC

CC C C

CC S

S SS

S

S SS

SS S S

S S