Don't Follow this Leader

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REFLECTION

One thing that we learned is that not knowing what all the primitive choices are makes programming hard! Once Nathan showed us "let" and "face" our NetLogo world was changed forever. We also, largely by trial and error, learned what kinds of primitives are reporters and which ones require agents and commands. This caused us to encounter various syntax and logic problems. One syntax challenge came up when we tried to call the enemy turtle to follow a command. Calling 'enemy', for example, reported a number rather than calling an agent, 'enemy'. We circumvented making the enemy red and the birds blue, and then calling turtles by their color. The code may be improved by calling an agent or agentset directly.

To change the model we created an enemy. All of the turtles have a "turtle's own" feature of enemy? true/false. You can change the number of enemies with a slider. The enemy just follows a straight path through the world and tells all the other turtles within it's vision to flee. The main logic problem we experienced related to developing a procedure to get the birds to avoid the enemy. First we prevented birds from flocking with the enemy by specifying that only blue turtles flock. We then adapted the 'flee' command to make birds avoid the enemy. We initially asked all birds to check whether the enemy was in their vision, and if it was, rotate 30 degrees. This made the model run very slowly and the enemy still ran into birds. To speed up the program, we reversed the direction of this commmand and had the enemy ask nearby birds to turn away. To get birds to avoid the enemy, we created a temporary set of birds that were close to the enemy and the enemy called them to 'face' the enemy and then turn 180 degrees. The enemy repels birds, which gives the appearance that birds avoid the enemy.

WHAT IS IT?

This model extends Uri's classic Flocking model. It shows the turtles flocking (or schooling) with enemies that float throught the world and make the turtles flee. Just like with Uri's original, the flocks that appear in this model are not created or led in any way by special leader birds. Rather, each bird is following exactly the same set of rules, from which flocks emerge.

HOW IT WORKS

The sliders determine the population of turtles and number of enemies. The enemies tell the other turtles when and how to flee.

The birds follow three rules: "alignment", "separation", and "cohesion".

"Alignment" means that a bird tends to turn so that it is moving in the same direction that nearby birds are moving.

"Separation" means that a bird will turn to avoid another bird which gets too close.

"Cohesion" means that a bird will move towards other nearby birds (unless another bird is too close).

When two birds are too close, the "separation" rule overrides the other two, which are deactivated until the minimum separation is achieved.

The three rules affect only the bird's heading. Each bird always moves forward at the same constant speed.

HOW TO USE IT

First, determine the number of birds you want in the simulation and set the POPULATION slider to that value. Then determine the number of enemies you want and set the ENEMY slider. Press SETUP to create the birds, and press GO to have them start flying around.

The default settings for the sliders will produce reasonably good flocking behavior. However, you can play with them to get variations:

Three TURN-ANGLE sliders control the maximum angle a bird can turn as a result of each rule.

VISION is the distance that each bird can see 360 degrees around it.

THINGS TO NOTICE

Central to the model is the observation that flocks form without a leader.

There are no random numbers used in this model, except to position the birds initially. The fluid, lifelike behavior of the birds is produced entirely by deterministic rules.

Also, notice that each flock is dynamic. A flock, once together, is not guaranteed to keep all of its members. Why do you think this is?

After running the model for a while, all of the birds have approximately the same heading. Why?

Sometimes a bird breaks away from its flock. How does this happen? You may need to slow down the model or run it step by step in order to observe this phenomenon.

THINGS TO TRY

Play with the sliders to see if you can get tighter flocks, looser flocks, fewer flocks, more flocks, more or less splitting and joining of flocks, more or less rearranging of birds within flocks, etc.

You can turn off a rule entirely by setting that rule's angle slider to zero. Is one rule by itself enough to produce at least some flocking? What about two rules? What's missing from the resulting behavior when you leave out each rule?

Will running the model for a long time produce a static flock? Or will the birds never settle down to an unchanging formation? Remember, there are no random numbers used in this model.

EXTENDING THE MODEL

Currently the birds can "see" all around them. What happens if birds can only see in front of them? The in-cone primitive can be used for this.

Is there some way to get V-shaped flocks, like migrating geese?

What happens if you put walls around the edges of the world that the birds can't fly into?

Can you get the birds to fly around obstacles in the middle of the world?

What would happen if you gave the birds different velocities? For example, you could make birds that are not near other birds fly faster to catch up to the flock. Or, you could simulate the diminished air resistance that birds experience when flying together by making them fly faster when in a group.

Are there other interesting ways you can make the birds different from each other? There could be random variation in the population, or you could have distinct "species" of bird.

NETLOGO FEATURES

Notice the need for the subtract-headings primitive and special procedure for averaging groups of headings. Just subtracting the numbers, or averaging the numbers, doesn't give you the results you'd expect, because of the discontinuity where headings wrap back to 0 once they reach 360.

RELATED MODELS

Moths
Flocking Vee Formation

CREDITS AND REFERENCES

This model is inspired by the Boids simulation invented by Craig Reynolds. The algorithm we use here is roughly similar to the original Boids algorithm, but it is not the same. The exact details of the algorithm tend not to matter very much -- as long as you have alignment, separation, and cohesion, you will usually get flocking behavior resembling that produced by Reynolds' original model. Information on Boids is available at http://www.red3d.com/cwr/boids/.

HOW TO CITE

If you mention this model in a publication, we ask that you include these citations for the model itself and for the NetLogo software:

Wilensky, U. (1998). NetLogo Flocking model. http://ccl.northwestern.edu/netlogo/models/Flocking. Center for Connected Learning and Computer-Based Modeling, Northwestern University, Evanston, IL.
Wilensky, U. (1999). NetLogo. http://ccl.northwestern.edu/netlogo/. Center for Connected Learning and Computer-Based Modeling, Northwestern University, Evanston, IL.

COPYRIGHT AND LICENSE

CC BY-NC-SA 3.0

This work is licensed under the Creative Commons Attribution-NonCommercial-ShareAlike 3.0 License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-sa/3.0/ or send a letter to Creative Commons, 559 Nathan Abbott Way, Stanford, California 94305, USA.

Commercial licenses are also available. To inquire about commercial licenses, please contact Uri Wilensky at uri@northwestern.edu.

This model was created as part of the project: CONNECTED MATHEMATICS: MAKING SENSE OF COMPLEX PHENOMENA THROUGH BUILDING OBJECT-BASED PARALLEL MODELS (OBPML). The project gratefully acknowledges the support of the National Science Foundation (Applications of Advanced Technologies Program) -- grant numbers RED #9552950 and REC #9632612.

This model was converted to NetLogo as part of the projects: PARTICIPATORY SIMULATIONS: NETWORK-BASED DESIGN FOR SYSTEMS LEARNING IN CLASSROOMS and/or INTEGRATED SIMULATION AND MODELING ENVIRONMENT. The project gratefully acknowledges the support of the National Science Foundation (REPP & ROLE programs) -- grant numbers REC #9814682 and REC-0126227. Converted from StarLogoT to NetLogo, 2002.

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Click to Run Model

turtles-own [
  flockmates         ;; agentset of nearby turtles
  nearest-neighbor   ;; closest one of our flockmates
  enemy?      ;; true or false: is this turtle an enemy?
]

to setup
  clear-all
  crt population
    [ set size 1.5  ;; easier to see
      setxy random-xcor random-ycor
      set enemy? false ] ;; because these are the good guys!
    crt enemy
     [ set size 3 ;; we want a big bad guy!
       setxy random-xcor random-ycor 
       set enemy? true ]   ;; all the other turtles will now act like this turtle is a bad guy.
    ask turtles [ recolor ]
  reset-ticks
end 

to flee  ;; turtle procedure
  ask turtles with [color = red][
  let close-turtles other turtles with [distance myself < vision]  ;; Nathan showed us how to use let to form a temporary agent set we're calling close-turtles
  ask close-turtles [set color yellow  face myself rt 180  ] ] ;; this is asking all the turtles close to the enemy to flash yellow and head the opposite direction from the enemy
end 



; color enemy turtle red, and keep the good guys blue

to recolor  ;; turtle procedure
  ifelse enemy?
    [ set color red ]
    [ set color blue ]
end 

to go
  ask turtles with [ color = blue ] [ flock ]  ;; this way only the regular birds flock, the enemy will not flock and attempt to chase the other birds, he will just cruise through the world following the fd 0.2 procedure
  ask turtles [ flee ]
  ;; the following line is used to make the turtles
  ;; animate more smoothly.
  repeat 5 [ ask turtles [ fd 0.2 ] display ]
  ask turtles [ recolor ] ;; this makes the birds that are near the enemy flash yellow but return to blue when they have fled
  tick
end 

to flock  ;; turtle procedure
  find-flockmates
  if any? flockmates
    [ find-nearest-neighbor
      ifelse distance nearest-neighbor < minimum-separation
        [ separate ]
        [ align
          cohere ] ] 
end 

to find-flockmates  ;; turtle procedure
  set flockmates other turtles in-radius vision
end 

to find-nearest-neighbor ;; turtle procedure
  set nearest-neighbor min-one-of flockmates [distance myself]
end 

;;; SEPARATE

to separate  ;; turtle procedure
  turn-away ([heading] of nearest-neighbor) max-separate-turn
end 

;;; ALIGN

to align  ;; turtle procedure
  turn-towards average-flockmate-heading max-align-turn
end 

to-report average-flockmate-heading  ;; turtle procedure
  ;; We can't just average the heading variables here.
  ;; For example, the average of 1 and 359 should be 0,
  ;; not 180.  So we have to use trigonometry.
  let x-component sum [dx] of flockmates
  let y-component sum [dy] of flockmates
  ifelse x-component = 0 and y-component = 0
    [ report heading ]
    [ report atan x-component y-component ]
end 

;;; COHERE

to cohere  ;; turtle procedure
  turn-towards average-heading-towards-flockmates max-cohere-turn
end 

to-report average-heading-towards-flockmates  ;; turtle procedure
  ;; "towards myself" gives us the heading from the other turtle
  ;; to me, but we want the heading from me to the other turtle,
  ;; so we add 180
  let x-component mean [sin (towards myself + 180)] of flockmates
  let y-component mean [cos (towards myself + 180)] of flockmates
  ifelse x-component = 0 and y-component = 0
    [ report heading ]
    [ report atan x-component y-component ]
end 

;;; HELPER PROCEDURES

to turn-towards [new-heading max-turn]  ;; turtle procedure
  turn-at-most (subtract-headings new-heading heading) max-turn
end 

to turn-away [new-heading max-turn]  ;; turtle procedure
  turn-at-most (subtract-headings heading new-heading) max-turn
end 

;; turn right by "turn" degrees (or left if "turn" is negative),
;; but never turn more than "max-turn" degrees

to turn-at-most [turn max-turn]  ;; turtle procedure
  ifelse abs turn > max-turn
    [ ifelse turn > 0
        [ rt max-turn ]
        [ lt max-turn ] ]
    [ rt turn ]
end 


; Copyright 1998 Uri Wilensky.
; See Info tab for full copyright and license.

There is only one version of this model, created about 12 years ago by Laurel Schrementi.

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