Flocks, Herds, and Schools: A Distributed Behavioral Model

1. Flocks, Herds, and Schools: A Distributed Behavioral Model By: Craig Reynolds Presented by: Stephanie Grosvenor

2. Flocks and Boids • Flock is a group of objects that exhibit the general class of polarized (aligned), non-colliding, aggregate motion • To simulate flock, simulate the behavior of individual bird • Perception • Flight dynamics • Boid is a simulated bird-like object

3. How do we simulate a flock in computer animation? • Scripting is bad • Particle system too simple • Make particle systems more complex • Local perception • Simulated physics • Flocking behaviors • Result of dense interaction of relatively simple behaviors of individually simulated birds

4. Behavioral Animation • Animator as a theatrical director • Character's performance is the indirect result of the director's instructions to the actor. • Not know how a simulation is going to proceed from the specified behaviors and initial conditions - many unexpected, pleasant surprises. • Problem getting uncooperative flocks to move as intended ("these darn boids seem to have a mind of their own!").

5. Boids and Turtles • Logo turtle • Movement forward, back, left, right • 3D turtles and their paths are equivalent to boid objects and their flight paths • Simple rules for complex behavior

6. Geometric Flight • Flight - Dynamic, incremental, rigid motion along a path • Geometrical transformation of an object, moving along and tangent to a 3D curve • Motion rigid - Underlying geometric model free to change shape within flying coordinate system

7. Geometric Flight • Conservation of momentum • Simple model of viscous speed damping – not exceed maximum speed • Minimum speed can be specified • Maximum acceleration used to provide smooth changes of speed and heading

8. Geometric Flight • Gravity modeled but only defines banking behavior • Many physical forces not supported

9. Banking • Magnitude of turning acceleration directly varies with object’s velocity and with the curvature of its path • Correct banking – object’s local space remains aligned with the perceptual or accelerational coordinate system • Makes boid fit viewer’s expectation of how flying objects should move and orient themselves

10. Natural Flocks, Herds, and Schools • Bird in flock must have behavior that allows it to coordinate movement with flockmates • Two balanced, opposing behaviors • Desire to stay close to flock • Desire to avoid collisions within flock • Individuals don’t pay much attention to each and every bird in flock • Bird’s perception of the rest of flock is localized and filtered • Itself • Two or 3 nearest neighbors • Rest of flock

11. Simulated Flocks • Collision Avoidance • Avoid collisions with nearby flockmates • Velocity Matching • Attempt to match velocity with nearby flockmates • Flock Centering • Attempt to stay close to nearby flockmates

12. Collision Avoidance • Urge to steer away from imminent impact • Static collision avoidance based on relative position of flockmates • Ignores velocity

13. Velocity Matching • Looks only at velocity • Ignores position • Predictive version of collision avoidance • If boid does a good job of matching velocity with neighbors – collision in near future unlikely • Static collision avoidance and dynamic velocity matching are complementary • Static collision avoidance serves to establish minimum required separation distance – velocity matching maintains it

14. Flock Centering • Boid localized perception of world center of flock actually means center of nearby flockmates • Causes boid to fly in a direction that moves it closer to centroid of nearby boids • If a boid is close to the center of the flock this will have little effect (since the boid density will be uniform), but if it is on the edges then it will have a greater effect. • Correctly allows simulated flocks to split

15. Arbitrating Independent Behaviors • three behavioral urges associated with flocking each expressed as acceleration requests • Behavior has several parameters – including strength, further accentuate the acceleration request • Navigation module of boid brain to collect relevant acceleration requests and then determine single behaviorally desired acceleration • Weighted average according to priority • Not good for collision with obstacles - Weighted average may cancel (opposite directions)

16. Arbitrating Independent Behaviors • Acceleration requests considered in priority order and added to accumulator • Magnitude added to another accumulator • Continue until sum of accumulated magnitudes larger than maximum acceleration value • Emergency acceleration allocated to satisfy pressing needs first • Example: Centering ignored in order to maneuver around obstacles

17. Simulated Perception • Perception model tries to model real bird • Filtering out surplus information that implements boid’s behavior • Real bird imperfect senses – nearby flockmates hide those farther away • Strongly localized information available • Aggregate motion of flocking depends on limited localized view of world

18. Each boid has direct access to the whole scene‘s geometric description, but flocking requires that it reacts only to flockmates within a certain small neighborhood around itself. The neighborhood is characterized by a distance (measured from the center of the boid) and an angle, measured from the boid's direction of flight. Flockmates outside this local neighborhood are ignored. The neighborhood defining the region in which flockmates influence a boids steering.

19. Scripted Flocking • Happy aimless flocking is good – direction better • Direct motion at specific times • Simulations under control of general-purpose animation scripting system • Interactive motion control facilities – provides ability to schedule invocation of user-supplied software on a frame by frame basis • Describe Timing of flock actions • Flexible Control over time varying values of parameters • Set up and animate all nonbehavioral aspects of scene background/lighting/camera motion/ visible objects

20. Scripted Flocking • Migratory urge – global target (global direction) • Global position – a target point toward which birds fly (going z for the winter) • Bounded acceleration, incrementally turns boid toward migratory target • Animate dynamic parameter – global position vector, global direction vector • Animate goal point along desired path

21. Avoiding Environmental Obstacles • Two types of shapes of environmental collision avoidance • Force field • Steer-to-avoid • Force Field • Field of repulsion force emanating from obstacle out into space • Boid increasingly repulsed as it gets closer to obstacle • Problem • approach obstacle surrounded by a force field at angle such that exactly opposite to direction of force field • Worst case scenario – fail to turn • Problem with peripheral vision • Too strong up close/ too weak far away • Want long-range planning to avoid obstacles, not panicky correction • Steer-to-avoid more robust - natural

22. Avoiding Environmental Obstacles • Steer-to-avoid • Obstacles that are directly in front of boid (where local z and obstacle intersect) • Silhouette edge of obstacle is closest to point of eventual impact • Radial vector computed – aim boid at point 1 body length beyond silhouette edge

23. Other Applications of the Flock Model Polarized, noncolliding aggregate motion • Traffic patterns, fish, locomotion, crowds • Scientific investigation

24. Questions?