boid - GAMA

Practice in GAML
BOIDS & BOIDS+
Alexis Drogoul - IRD - France
Presentation of the model
Design of the model
Implementation in GAML
Additional features

2
BOIDS: Introduction
• Introduced in 1987 by Craig Reynolds as a model of the flocking (or herding
or schooling) behaviours observed with intelligent life-forms, especially birds.
• http://www.red3d.com/cwr/boids/
• Used for film animations (like The Lion King, or The Lord of the Rings)
• BOIDS comes from “bird”+“oid” (like android, for instance).

Flocks and Boids
• A flock exhibits polarized (aligned), non-colliding, aggregate motion. Motion is
fluid, random, synchronized and gives an impression of intention and control
• Yet, biological evidence: the collective motion is the aggregate result of
individual actions of birds and not the result of a central control
• The idea of Reynods was to simulate flocks by modeling the behavior of
birds : local perception of the world, local interactions with other birds
• A boid is then a bird-like agent (and the flock a multi-agent system).

4
Original Boids Model
• Interactions between boids take place within a local neighborhood of each
boid.
• Three basic steering (navigation) behaviors:
◐ Separation
◐ Alignment
◐ Cohesion
• Each steering behavior directs the boid in a desired direction.
• The directions are then combined (averaged, with or without weight, or
prioritized) by the “brain” of the boid to obtain the final direction of flight.

Flocking – Steering Behaviors (1)
• Separation: boids steer to avoid crowding local flockmates.
◐ Collision avoidance.
◐ Keeps boids a realistic distance apart.

6
Flocking – Steering Behaviors (2)
• Alignment (or Velocity Matching): boids attempt to match the velocity
(direction, speed) of their neighbors.
◐ Complements separation.
◐ Causes boids to move in the same general direction.

7
Flocking – Steering Behaviors (3)
• Cohesion: boids steer to move towards the average position of local
flockmates.
◐ Causes boids to keep together in a local flock.
◐ Allows flocks to both merge and bifurcate.

8
Implementing it in GAML
• Boids are initially placed randomly
◐ What init procedure ?
• Boids are able to move and perceive their environment (2D)
◐ What skills ?
• Boids will chose their direction depending on their perception
◐ Which perception ?
• Boids apply the three previous rules to make their choice (see pseudo-code
after)
◐ Which reflexes/actions ?

9
Pseudo-code
• These rules can be written in pseudo-code like:
BOID.PROCEDURE move()
Vector v1, v2, v3
percept = perception()
v1 = cohesion(percept)
v2 = separation(percept)
v3 = alignment(percept)
velocity =velocity +v1 +v2 +v3
location = location +velocity
END PROCEDURE
BOID.PROCEDURE cohesion(percept)
Vector mc
FOR EACH Boid b in percept
mc = mc + b.location
END
mc = mc / size(percept)
RETURN (mc - location) / CF
END PROCEDURE
BOID.PROCEDURE alignment(percept)
Vector pv
FOR EACH Boid b in percept
pv = pv + b.velocity
END FOR
pv = pv / size(percept)
RETURN (pv - velocity) / AF
END PROCEDURE
BOID.PROCEDURE separation(percept)
Vector c
FOR EACH Boid b in percept
IF |b.location-location| < SF
c = c-(b.location-location)
END FOR
RETURN c
END PROCEDURE

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BOIDS+
1.Apply a boundary to prevent the birds from “escaping” the environment (or
getting stuck on its borders). Same kind of rule as before:
BOID.PROCEDURE bound()
Vector V
IF location.x < BF
v = v + {BF,0}
IF location.x > env_size.x - BF
v = v - {BF,0}
... Etc...
RETURN V
END PROCEDURE
2.Add a goal that all birds will try to reach
◐ The goal is a point randomly fixed by the world, changed everytime a bird is
close to it
BOID.PROCEDURE goal()
RETURN (goal - location) / GF
END PROCEDURE

11
BOIDS+
3.Apply a limit on the angle (AL) that a bird can turn each step
◐ Idea: use the min and max properties of the heading variable
4.Transform AF, BF, CF, GF, SF, and AL into global parameters
◐ (and play with them!)
5.Add a new species of agents: obstacles.
◐ Placed at random, immobile, cannot be “crossed” by birds
◐ Add the necessary rule in birds
◐ The flocks should split and reform with certain parameter sets (like “real birds”)
6.Make the obstacles move slowly towards birds
• Add the necessary parameters to the world to control them