Biology: flocking, herding & schooling Day 5 COLQ 201 Multiagent modeling Harry Howard Tulane University

22-Jan-2010COLQ 201, Prof. Howard, Tulane University2 Course organization   Photos?

22-Jan-2010COLQ 201, Prof. Howard, Tulane University3 Photos

Boids

22-Jan-2010COLQ 201, Prof. Howard, Tulane University5 What did you learn about Boids?  Date?  First appearance?  Movies?  A-life?

22-Jan-2010COLQ 201, Prof. Howard, Tulane University6 Steering behaviors  They describe how an individual boid maneuvers based on the positions and velocities its nearby flockmates:  separation  alignment  cohesion

22-Jan-2010COLQ 201, Prof. Howard, Tulane University7 Separation  Steer to avoid crowding local flockmates.

22-Jan-2010COLQ 201, Prof. Howard, Tulane University8 Alignment  Steer towards the average heading of local flockmates.

22-Jan-2010COLQ 201, Prof. Howard, Tulane University9 Cohesion  Steer to move toward the average position of local flockmates.

22-Jan-2010COLQ 201, Prof. Howard, Tulane University10 Neighborhood  Distance (measured from the center of the boid) and  Angle, measured from the boid's direction of flight.  It could be considered a model of  limited perception (as by fish in murky water)  the region in which flockmates influence a boid's steering.

MyFlocking Community model

22-Jan-2010COLQ 201, Prof. Howard, Tulane University12 Overview  What do you see in the interface?  How does it compare to Boids?

22-Jan-2010COLQ 201, Prof. Howard, Tulane University13 Questions  Keeping the other parameters at their default values (vision = 3, min-separation = 1, max-align- turn = 5, max-cohere-turn = 3, max-separate-turn = 1.5), …  what does vision do?  what does minimum-separation do?  what does max-align-turn do?  what does max-cohere-turn do?  what does max-separate-turn do?

Conclusions

22-Jan-2010COLQ 201, Prof. Howard, Tulane University15 Chaos and emergence  In Boids (and related systems) interaction between simple behaviors of individuals produce complex yet organized group behavior.  The component behaviors are inherently nonlinear, so mixing them gives the emergent group dynamics a chaotic aspect.  At the same time, the negative feedback provided by the behavioral controllers tends to keep the group dynamics ordered.  The result is life-like group behavior.

22-Jan-2010COLQ 201, Prof. Howard, Tulane University16 Time scales  A significant property of life-like behavior is unpredictability over moderate time scales.  At very short time scales, the motion is quite predictable: one second from now a boid will be traveling in approximately the same direction.  Yet if the boids are flying primarily from left to right, it would be all but impossible to predict which direction they will be moving (say) five minutes later.

22-Jan-2010COLQ 201, Prof. Howard, Tulane University17 At the edge of chaos  This property is unique to complex systems and contrasts with both random behavior (which has neither short nor long term predictability) and ordered behavior (which is predictable in both the short and long term).  This fits with Langton's 1990 observation that life- like phenomena exist at the edge of chaos.

22-Jan-2010COLQ 201, Prof. Howard, Tulane University18 Chaos

22-Jan-2010COLQ 201, Prof. Howard, Tulane University19 Agents  Boids is an example of an individual-based model, a class of simulation used to capture the global behavior of a large number of interacting autonomous agents.  Individual-based models are being used in biology, ecology, economics and other fields of study (and in this course).

22-Jan-2010COLQ 201, Prof. Howard, Tulane University20 Complexity  A straightforward implementation of the boids algorithm has an asymptotic complexity of O(n 2 ).  Each boid needs to consider every other boid, if only to determine whether it is a nearby flockmate.  However it is possible to pare this cost down to nearly O(n) by the use of a suitable spatial data structure which allows the boids to be kept sorted by their location.  Finding the nearby flockmates of a given boid then requires examining only the portion of the flock which is within the general vicinity.

Programming NetLogo

22-Jan-2010COLQ 201, Prof. Howard, Tulane University22 The NetLogo world  … is a two dimensional world that is made up of turtles, patches and an observer.  The patches create the ground in which the turtles can move around on and  the observer is a being that oversees everything that is going on in the world.

22-Jan-2010COLQ 201, Prof. Howard, Tulane University23 P1  I will ask you to open a model that you have not seen, and I will ask you to answer some questions about it and how it works.

22-Jan-2010COLQ 201, Prof. Howard, Tulane University24 Next time  P1  Biology: from foraging to graph theory  Ants2, AntSystem