Agents and Avatars 2

Ruth Aylett

Overview

!  Crowds !  Speech and Expressive behaviour

– Embodied conversational characters

!  Creating autonomy – Scripting – Architectures

Crowds and flocking

!  Interactions among members of a group –  Local neighborhood

!  Reynolds Boids –  Originally birds or

fish –  Complex behaviour

determined by simple rules

Separation: Boid Avoidance

Alignment

Cohesion

Motor Control

!  Steering Force !  Integrate to determine acceleration

–  Thrust – determines speed

–  Lateral Steering Force – determines direction

Boid Object Representation

!  Point Mass Vehicle –  Mass –  Position –  Velocity –  Orientation

•  Constrained to align with velocity

–  Force and Speed Limits

!  (No moment of inertia)

Same maths as point objects

acceleration = steering_force / mass – Given f = ma

velocity = velocity + acceleration – Over a unit time period – Generically v = v x at

position = position + velocity – Over a unit time period – Generically s = s + vt

Seeking and Fleeing !  Aim towards target

Desired_velocity = Kp (position – target) Steering = desired_velocity – velocity

Seeking and Fleeing Applet (Reynolds)

Pursuing and Avoiding

!  Target is another moving object !  Predict target’s future position !  Scale prediction time, T, based on distance to object, Dc T=Dc

Pursuing and avoiding applet (Reynolds)

More Behaviors

!  Evasion –  Like flee, but predict pursuer’s movement

!  Arrival –  Like seek, but stop at target –  Applet (Reynolds)

!  Obstacle Avoidance –  Repulsive force –  Aim to boundary –  Adjust velocity to be perpendicular to surface normal

Do People Flock?

Social psychologist’s report that people tend to travel as singles or in groups of size 2 to 5.

“Controlling Steering Behavior for Small Groups of Pedestrians in Virtual Urban Environments”

Terry Hostetler, Phd dissertation, 2002

Characteristics of Small Groups

!  Proximity !  Coupled Behavior !  Common Purpose !  Relationship Between

Members

Moving Formations

!  Pairs: Side by side !  Triples: Triangular shape

Stationary Formations

Moving pair approaches stationary triple Stationary quintuple

formed

!  Two Parameters - because in 2D – Acceleration

•  Increase/reduce walking speed • Combination of step length and step rate

– Turn • Adjust orientation • Heading direction for forward walking

Locomotion Model for Walking

Avoiding an Obstacle -- Trajectory

Small look-ahead distance Large look-ahead distance

ped 1

ped 2

walkway axis walkway axis

ped 1

ped 2

Interaction Between Pairs -- 1

Interaction Between Pairs -- 2

Interaction Between Pairs -- 3

Adding goals

!  People are usually going somewhere – Easy case: same goal

• Walking to a sports stadium • Political demonstrations

– Harder case: multiple goals • Campus traffic • Most public spaces: streets, shops, stations

Talking Heads

!  Human faces – Thus high expectations:

• Blinking, random head movement •  Facial muscles •  Lip sync

!  Rarely intelligent – Entirely scripted in most cases – No integration of speech production with

intelligent architecture and NL generation

Moving the face - 1

!  Simple approach – Define overlay frames with different facial

expressions •  For example, six mouth shapes

– Use signal from TTS engine to pick a mouth shape

– This works OK with cartoon-like characters

Moving the face - 2

!  Visemes –  Mouth and lip position

for each phoneme –  Correspond to a

submesh in the geometry

Using visemes

!  Find viseme for each phoneme – Synchronise graphical change with sound

Catalan weather forecaster

!  Complete talking character – Weather forecasts have predictable content – Still have to match content and behaviour

• <demo via browser: sam_mosca_ok.wmv>

Autonomy

!  An independent sense-reflect-act cycle: implies –  An agent-based action-selection mechanism –  Self-animation in real-time: cannot be pre-rendered –  Local sensing –  Virtual robotics

!  Not a natural graphics view –  Global manipulation of polygons –  Hierarchical scene-graph

Scripting

!  Typical of NPCs in computer games !  What representational content is

needed? – Can just invoke named animations – What parameters?

• Depends on animation approach

Scripting with Finite State Automata

Tennis game FSM sequence"

Limitations of scripting

!  Becomes predictable – Either independent of local sensing – Or single FSM transition stimulus

!  Limited ability to use internal state –  Interaction memory – Affective state

!  Lacks sequencing flexibility – Sequence is hardwired

Low-level architecture for a fish…

Functional Architecture

High-level architecture

VE or visualisation?

!  Embed all behaviour into scenegraph? – Advantage: efficiency – Disadvantage: tied to scenegraph software

!  Or visualise simulated world? – Loosely coupled: how to link? – Message-passing between world simulation

and scenegraph

Using game engines

!  Offer some support for NPCs –  More facilities than a scenegraph –  Attaching animations to graphical bodies –  Imports from popular 3D packages like Character

Studio

!  Disadvantages –  Proprietary; oriented to scripting –  Hard (in some cases impossible) to implement sensing

•  Fixed paths quite often

–  Often focused on shooting