[PDF] - 9.4 Local Perception Filters 9.4 Local Perception Filters PDF Document

SLIDE 1

1 §9.4 Local Perception Filters §9.4 Local Perception Filters

exploiting human’s perceptual limitations

exploiting human’s perceptual limitations

level

level-

of
f-
detail: less details where they cannot be observed

detail: less details where they cannot be observed

image, video and audio compression

image, video and audio compression

local perception filters

local perception filters

exploits temporal perception

exploits temporal perception

shows possibly out

shows possibly out-

of
f-
date information (≠ dead reckoning)

date information (≠ dead reckoning)

ensures consistent interaction

ensures consistent interaction

allows to introduce artificial delays (e.g., bullet time)

allows to introduce artificial delays (e.g., bullet time)

Exploiting Exploiting Perceptual Limitations Perceptual Limitations

Humans

Humans have have inherent perceptual limitations inherent perceptual limitations Two approaches to exploit Two approaches to exploit

Information can provided at multiple levels of detail and at

Information can provided at multiple levels of detail and at different update rates different update rates

Mask the timeliness characteristics of information

Mask the timeliness characteristics of information

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

SLIDE 2

2 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

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

Local Perception Filters (LPFs) Local Perception Filters (LPFs)

introduced by Sharkey, Ryan & Roberts (1998)

introduced by Sharkey, Ryan & Roberts (1998)

a method for hiding communication delays in networked

a method for hiding communication delays in networked virtual environments virtual environments

exploits the human perceptual limitations by rendering entities

exploits the human perceptual limitations by rendering entities slightly out slightly out-

of
f-
date locations based on the underlying network

date locations based on the underlying network delays delays

causality of events is preserved

causality of events is preserved

rendered view may have temporal distortions

rendered view may have temporal distortions

rendered view ≠ real view

rendered view ≠ real view

Active and Passive Entities Active and Passive Entities

An active

An active entity (i.e., player) entity (i.e., player)

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

SLIDE 3

3 Rules of LPFs Rules of LPFs

1. 1.

Player should be able to interact in real Player should be able to interact in real-

time with the nearby

time with the nearby entities. entities.

2. 2.

Player should be able to view remote interactions in real Player should be able to view remote interactions in real-

time, although they can be out

time, although they can be out-

of
f-
date.

date.

3. 3.

Temporal distortions in the player’s perception should be as Temporal distortions in the player’s perception should be as unnoticeable as possible. unnoticeable as possible.

p p n n r r q q

Interaction Between Players Interaction Between Players

interaction = communication between the players

interaction = communication between the players

local players: immediate

local players: immediate

remote players: subject to the network latency

remote players: subject to the network latency

time frame = current time

time frame = current time – – communication delay communication delay

interaction = players exchanging passive entities

interaction = players exchanging passive entities

passive entities are predictable

passive entities are predictable ⇒ ⇒ they can be rendered in the past (or in the they can be rendered in the past (or in the future) future)

a passive entity can change its time frame dynamically

a passive entity can change its time frame dynamically

the nearer to a local player, the closer it is rendered to the c

the nearer to a local player, the closer it is rendered to the current time urrent time

the nearer to a remote player, the closer it is rendered to its

the nearer to a remote player, the closer it is rendered to its time frame time frame

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

The View The View of

f the Blue

the Blue Player Player

t t

The View The View of the

f the Red

Red Player Player

t t

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

SLIDE 4

4 3 3½ ½-

Dimensional

Dimensional Temporal Contour Temporal Contour

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) )