Behavior

1
Behavior
2
Autonomous Characters

• Acknowledgement
• Much 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

3
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

4
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.

5
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.

6
Three-Tier Hierarchy

• Action selection
• goals and strategies What to do
• Steering
• guidance / motion control How to do it
• Locomotion
• movement generation Getting it
  done

7
Cowboy Analogy

• Action selection
• Trail boss Fetch that stray.
• Steering
• Cowboy Giddy-up, that away.
• Locomotion
• Horse Wilbur!

8
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.

9
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.

10
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.

11
Motor Control

• Steering Force
• Integrate to determine acceleration
• Thrust determines speed
• Lateral Steering Force determines direction

12
Boid Object Representation

• Point Mass Vehicle
• Mass
• Position
• Velocity
• Orientation
• Constrained to align with velocity
• Force and Speed Limits
• (No moment of intertia)

13
Euler Integration

• acceleration steering_force / mass
• velocity velocity acceleration
• position position velocity

14
Seeking and Fleeing

• Aim towards target
• Desired_velocity Kp (position target)
• Steering desired_velocity velocity

Seeking and Fleeing Applet (Reynolds)
15
Pursuing and Avoiding

• Target is another moving object
• Predict targets future position
• Scale prediction time, T, based on distance to
  object, Dc
• TDc

Pursuing and avoiding applet (Reynolds)
16
More Behaviors

• Evasion
• Like flee, but predict pursuers movement
• Arrival
• Like seek, but step at target
• Applet (Reynolds)
• Obstacle Avoidance
• Repulsive force
• Aim to boundary
• Adjust velocity to be perpendicular to surface
  normal

17
Flocking Behaviors

• Interactions among members of a group
• Local neighborhood

18
Separation Boid Avoidance
19
Alignment
20
Aggregation
21
Leader Following

• Based on arrival
• Target is behind leader
• Clear leaders front
• Separation avoids crowding
• Applet (Reynolds)

22
Arbitration of Competing Demands

• State Machines
• Context dependent selection
• Problem combinatorial explosion
• Winner Take All
• Choose highest priority goal
• Problems dithering, fairness, and tunnel vision
• Blending
• Combine output (e.g. sum, average, min, )
• Problem combination may satisfy no one

23
Flocking Demos

• Flocking Applet (Craig Reynolds)
• Fish Schooling (Steve Hughes)
• Beach House (Ishihama Yoshiaki )
• For more demos see Reynolds Boids in Java

24
Do People Flock?
Social psychologists 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
25
Characteristics of Small Groups

• Proximity
• Coupled Behavior
• Common Purpose
• Relationship Between Members

26
Moving Formations

• Pairs Side by side
• Triples Triangular shape

27
Stationary Formations
Stationary quintuple formed
Moving pair approaches stationary triple
28
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

29
Action Space
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
30
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)

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Vote Tabulation
Pursuit Point Tracking
Maintain Target Velocity
Inertia
1.0
Avoid Peds
Maintain Formation
Avoid Obstacles
1.0
1.0
Centering
4.0
2.0
5.0
2.0
Electioneer
Winning Cell
32
A Group of Two Following a Path
Pursuit Point Tracking
Maintain Formation
walkway axis
-1.0 -1.0 1.0 -1.0 -1.0 1.0 -1.0 -1.0
1.0
1.0 1.0 1.0 -1.0 -1.0 -1.0 -1.0 -1.0
-1.0
pursuit point ?
2.0
1.0
1.0 1.0 3.0 -3.0 -3.0 -1.0 -3.0 -3.0
-3.0
Election for ped 1
?
ped 2
?
ped 1
Winning vote Accelerate/Turn Right
33
Avoiding an Obstacle -- Trajectory
walkway axis
walkway axis
ped 1
ped 1
ped 2
ped 2
Small look-ahead distance
Large look-ahead distance
34
Interaction Between Pairs -- 1
35
Interaction Between Pairs -- 2
36
Interaction Between Pairs -- 3
37
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)

38
Legged Motion

• Statically Stable Walking
• Dynamically Stable Running
• Legged robots that balance
• by Marc H. Raibert (1986)
•    ISBN0-262-18117-7
• Also Legged Robots
• by Marc Raibert, CACM, V. 6, N.
  29, pp. 499-514
• June 1986,
• (See me for class notes)