Exploiting Level- Exploiting Level -of of- -Detail Perception - - PDF document

SLIDE 1

Special Course on Networked Virtual Environments February 26, 2004 Jouni Smed 1

Exploiting Level Exploiting Level-

• of
• f-
• Detail Perception

Detail Perception

• Nearby viewers

Nearby viewers

• expect full graphical

expect full graphical details details

• accurate structure, position, orientation

accurate structure, position, orientation

• update rate

update rate → → local frame rate local frame rate

• Distant viewers

Distant viewers

• can tolerate

can tolerate less less graphical graphical details details

• less accurate structure, position, orientation

less 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 channel

resolution channel

( (x x, , y y) ) ( (x x, , y y) )

High High-

• resolution channel

resolution channel

High High-

• frequency,

frequency, high high-

• bandwidth

bandwidth information information Low Low-

• frequency

frequency, , low low-

• bandwidth

bandwidth 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 channel

each transmission identifies its channel

• server dispatches data from channels to clients

server dispatches data from channels to clients

• Multicast group for each region

Multicast group for each region

• assign multiple

assign multiple addresses addresses for each region for each region

• ne group provides all of the entities’ high
• ne 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 channels

resolution channels

• Multicast group for each entity

Multicast group for each entity

• assign multiple

assign multiple addresses addresses for each entity for each entity

• Different reliabilities to each channel

Different reliabilities to each channel

• low

low-

• frequency updates are

frequency updates are important important

• lost packets can have a significant impact

lost 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
• ff, three channels for each entity

, three channels for each entity is typically needed is typically needed

• channels

channels provide order provide order-

• of
• f-
• magnitude differences in

magnitude differences in

• structural and positional accuracy

structural and positional accuracy

• packet rate

packet rate Rigid Rigid-

• body channel

body channel Approximate Approximate-

• body channel

body channel Full Full-

• body channel

body channel Far Far-

• range viewers

range viewers Mid Mid-

• range viewers

range viewers Near Near-

• range viewers

range 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:

• position

position

• rientation
• rientation
• structure

structure

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 parts

appendages and other articulated parts

• Provided information is entity

Provided information is entity-

• specific

specific

• corresponds

corresponds to the dominant changes of the structure to the dominant changes of the structure

SLIDE 2

Special Course on Networked Virtual Environments February 26, 2004 Jouni Smed 2

Common Approximations Common Approximations

• Radial length

Radial 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 vector

Articulation 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 co

Local co-

• ordinate system points
• rdinate 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 system
• rdinate 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

• viewer

viewer can subscribe to a limited number of full can subscribe to a limited number of full-

• body

body channels channels

• 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-

• f
• f-
• 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: :

• works

works on

• n WANs

WANs having great latency having great latency

• can

can enhance packet aggregation enhance packet aggregation

• can

can enhance dead reckoning enhance dead reckoning

Active and Passive Entities Active and Passive Entities

• An active entity

An active entity

• takes actions on its own

takes actions on its own

• generates updates

generates updates

• human participants, computer

human participants, computer-

• controlled entities

controlled entities

• cannot be predicted typically

cannot be predicted typically

• rendered using state updates

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

• A passive entity

A 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

• f its nearest active entity
• f its nearest active entity
• reacts

reacts instantaneously to instantaneously to the the actions actions of

• f a nearby

a nearby active entity active 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

SLIDE 3

Special Course on Networked Virtual Environments February 26, 2004 Jouni Smed 3

View View of the

• f 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 time

the current time

• meaningful interaction for local player

meaningful interaction for local player

• a past time reference

a past time reference

• network latency

network latency

• bserving meaningful interaction for remote
• bserving meaningful interaction for remote player

player

3 3½ 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
• rdinate system (

system (x x, , y y, , z z, , t t) )

• x

x, , y y, , z z: 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, 0 0, 0, 0) )

• t

t: 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

• pposing player:
• pposing player: t

t = − = −d d

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

d d

Co Co-

• ordinate
• rdinate Systems

Systems

t t = 0 = 0 t t = − = −d d

Blue Player Blue Player

t t = − = −d d t t = 0 = 0

Red Player Red Player

Properties of the Co Properties of the Co-

• ordinate System
• rdinate System
• The co

The co-

• ordinate system is defined
• rdinate 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

• Two interacting objects are

Two interacting objects are rendered at the same time rendered at the same time reference point reference point

• 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 time time

• Smooth movement

Smooth movement

Generalizing the Local Temporal Contour Generalizing the Local Temporal Contour

• Limitations:

Limitations:

• players are capable of moving along a single axis only

players are capable of moving along a single axis only

• supports two

supports two active objects only active objects only

• Generalization to a 4D

Generalization to a 4D co co-

• ordinate
• rdinate system

system requires preserving requires preserving for the local user: for the local user:

• interacting

interacting naturally with naturally with passive objects passive objects in vicinity in vicinity

• seeing

seeing remote interactions remote interactions (passive (passive-

• to

to-

• passive,

passive, passive passive-

• to

to-

• active)

active) naturally naturally

• perceiving

perceiving smooth motion of remote objects smooth motion of remote objects

SLIDE 4

Special Course on Networked Virtual Environments February 26, 2004 Jouni Smed 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

• ver

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 averaged averaged latency to dampen the effect latency 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 reckoning

predict entity’s past position, dead reckoning

• Computational requirements

Computational requirements

• compute

compute the contour the contour using only the nearest active entities using only the nearest active entities

§6.4 Enhancing the System Architecture §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 clusters

server clusters

• partition clients across multiple servers

partition clients across multiple servers

• partition the NVE across multiple servers

partition the NVE across multiple servers

• server hierarchies

server hierarchies

• peer

peer-

• server systems

server systems

M M H H B B T T P P

Traditional Client Traditional Client-

• Server

Server

• Server may act as

Server may act as

• broadcast reflector

broadcast reflector

• filtering reflector

filtering reflector

• packet aggregation server

packet aggregation server

• Scalability problems

Scalability problems

• all traffic

all traffic goes through goes through the the server server

⇒ ⇒Server clusters Server clusters

S S C C C C C C C C C C C C C C C C C C C C C C C C 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 interests

inform the interests of their

• f 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 S S S S C C C C C C C C C C C C C C C C C C C C C C C C C C C C

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 traffic

Eliminates a lot of network traffic

• Requires

Requires advanced advanced configuration configuration

• Is

Is a region a region visible from another visible from another region? region?

• Aggregation

Aggregation servers are servers are a special a special case of NVE server partitioning case of NVE server partitioning

C C C C C C C C C C C C C C S S S S S S S S

SLIDE 5

Special Course on Networked Virtual Environments February 26, 2004 Jouni Smed 5

Server Hierarchies Server Hierarchies

• Servers themselves

Servers themselves act as act as clients clients

• Packet from

Packet from an upstream an upstream server: server:

• deliver to

deliver to the interested the interested downstream clients downstream clients

• Packet from

Packet from a downstream a downstream client: client:

• deliver to

deliver to the interested the interested downstream clients downstream clients

• if other regions are interested in

if other regions are interested in the the packet packet then then deliver it deliver it to to the the upstream upstream server server

C C C C C C C C C C C C C C S S S S S S S S S S S S S S