Behavior

Autonomous Characters

AcknowledgementMuch of this material is taken from the work of Craig Reynolds. He maintains a web pages including a rich source of material of steering behavior and the consumate source on flocking.

Also see: Steering Behaviors For Autonomous Characters

by Craig Reynolds

Autonomous Characters

Self-Directed characters "puppets that pull their own strings" -Ann Marion Situated Live in a world shared by other entities Embodied Physical manifestation (virtual) Reactive

instinctive, driven by stimulus Improvisation, life-like behavior

Emergent Behavior

The appearance of consistent global behavior from a set of local rules enforcing independent constraints.

Emergent group behavior is the appearance of coordinated collective behavior of many individuals from individual behaviors based on independent, local interactions.

Emergent Misbehavior?

Permits modular development of complex behaviors

Hard to predict interactions among rules• Sometimes surprising and undesirable behaviors

appear in new circumstances or when new rules are added.

• Hard to debug.

Three-Tier Hierarchy

Action selectiongoals and strategies “What to do”

Steeringguidance / motion control “How to do it”

Locomotionmovement generation “Getting it done”

Cowboy Analogy

Action selection Trail boss: “Fetch that stray.”

SteeringCowboy: “Giddy-up, that away.”

LocomotionHorse “Wilbur!”

Flocks in Film

1987: Stanley and Stella in: Breaking the Ice, (short) Director: Larry Malone, Producer: Symbolics, Inc.

1988: Behave, (short) Produced and directed by Rebecca Allen

1989: The Little Death, (short) Director: Matt Elson, Producer: Symbolics, Inc.

1992: Batman Returns, (feature) Director: Tim Burton, Producer: Warner Brothers

1993: Cliffhanger, (feature) Director: Renny Harlin, Producer: Carolco.

1994: The Lion King, (feature) Director: Allers / Minkoff, Producer: Disney.

Flocks in Film

1996: From Dusk Till Dawn, (feature) Director: Robert Rodriguez, Producer: Miramax

1996: The Hunchback of Notre Dame, (feature) Director: Trousdale / Wise, Producer: Disney.

1997: Hercules, (feature) Director: Clements / Musker, Producer: Disney.

1997: Spawn, (feature) Director: Dipp₫, Producer: Disney.

1997: Starship Troopers, (feature) Director: Verhoeven, Producer: Tristar Pictures.

1998: Mulan, (feature) Director: Bancroft/Cook, Producer: Disney.

Flocks in Film

1998: Antz, (feature) Director: Darnell/Guterman/Johnson, Producer: DreamWorks/PDI.

1998: A Bugs Life, (feature) Director: Lasseter/Stanton, Producer: Disney/Pixar.

1998: The Prince of Egypt, (feature) Director: Chapman/Hickner/Wells, Producer: DreamWorks.

1999: Star Wars: Episode I--The Phantom Menace, (feature) Director: Lucas, Producer: Lucasfilm.

2000: Lord of the Rings: the Fellowship of the Ring (feature) Director: Jackson, Producer: New Line Cinema.

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 intertia)

Euler Integration

acceleration = steering_force / mass

velocity = velocity + acceleration

position = position + velocity

Seeking and Fleeing

Aim towards targetDesired_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 step at target Applet (Reynolds)

Obstacle Avoidance1. Repulsive force2. Aim to boundary3. Adjust velocity to be perpendicular to surface

normal

Flocking Behaviors

Interactions among members of a group

Local neighborhood

Separation: Boid Avoidance

Alignment

Aggregation

Leader Following

Based on arrival Target is behind leader

Clear leader’s front Separation avoids

crowding Applet (Reynolds)

Arbitration of Competing Demands

1. State Machines Context dependent selection Problem: combinatorial explosion

2. Winner Take All Choose highest priority goal Problems: dithering, fairness, and tunnel vision

3. Blending Combine output (e.g. sum, average, min, …) Problem: combination may satisfy no one

Flocking Demos

Flocking Applet (Craig Reynolds) Fish Schooling (Steve Hughes) Beach House (Ishihama Yoshiaki )

For more demos see Reynolds “Boids in Java”

Do People Flock?

Social psychologist’s report the 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 approachesstationary triple

Stationary quintuple formed

Locomotion Model for Walking

Two Parameters Acceleration

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

Turn Adjust orientation Heading direction for forward walking

Accelerate Accelerate Accelerate Turn Left No Turn Turn Right

Coast Coast Coast Turn Left No Turn Turn Right

Decelerate Decelerate Decelerate Turn Left No Turn Turn Right

Action Space

Distributed Preference Voting

Seek best compromise through democratic voting Delegation of voters: Constraint Proxies

Proxies vote on every possible value of control variable (Weighed) votes are tallied

“Some citizens are more equal than others”

(Who said life was fair?) Winning cell represents best compromise

Bias towards incumbents to reduce dithering

(Now this is REAL politics)

Vote Tabulation

1.0

Pursuit Point

Tracking

Maintain Formation

Inertia

Centering

Maintain Target

Velocity

Avoid Peds

Winning Cell

Electioneer

1.0

1.0

2.0

2.0

4.0

5.0

Avoid Obstacles

A Group of Two Following a Path

ped 1

walkway axis

pursuit point

Winning vote = Accelerate/Turn Right

ped 2

-1.0 -1.0 +1.0-1.0 -1.0 +1.0-1.0 -1.0 +1.0

Pursuit Point Tracking

+1.0 +1.0 +1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0

Maintain Formation

+1.0 +1.0 +3.0 -3.0 -3.0 -1.0 -3.0 -3.0 -3.0

2.01.0

Election for ped 1

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

Motion Control Through Optimization

Space-Time Constraints a great place to start is the Witkin and Kass SIGGRAPH paper

Spacetime Constraints

Andrew Witkin and Michael Kass,

SIGGRAPH, V. 22, N. 4, pp. 159-168, 1988.

(See me for class notes)

Legged Motion

Statically Stable Walking Dynamically Stable Running Legged robots that balance

by Marc H. Raibert (1986)

   ISBN:0-262-18117-7

Also: Legged Robots

by Marc Raibert, CACM, V. 6, N. 29, pp. 499-514

June 1986,

(See me for class notes)