Flocking

Flocking by Daniel Shiffman.

An implementation of Craig Reynold’s Boids program to simulate the flocking behavior of birds. Each boid steers itself based on rules of avoidance, alignment, and coherence.

Click the mouse to add a new boid.


Flock flock;

void setup() {
  size(640, 360);
  flock = new Flock();
  // Add an initial set of boids into the system
  for (int i = 0; i < 150; i++) {
    flock.addBoid(new Boid(width/2,height/2));
  }
}

void draw() {
  background(50);
  flock.run();
}

// Add a new boid into the System
void mousePressed() {
  flock.addBoid(new Boid(mouseX,mouseY));
}



// The Flock (a list of Boid objects)

class Flock {
  ArrayList<Boid> boids; // An ArrayList for all the boids

  Flock() {
    boids = new ArrayList<Boid>(); // Initialize the ArrayList
  }

  void run() {
    for (Boid b : boids) {
      b.run(boids);  // Passing the entire list of boids to each boid individually
    }
  }

  void addBoid(Boid b) {
    boids.add(b);
  }

}




// The Boid class

class Boid {

  PVector position;
  PVector velocity;
  PVector acceleration;
  float r;
  float maxforce;    // Maximum steering force
  float maxspeed;    // Maximum speed

    Boid(float x, float y) {
    acceleration = new PVector(0, 0);

    // This is a new PVector method not yet implemented in JS
    // velocity = PVector.random2D();

    // Leaving the code temporarily this way so that this example runs in JS
    float angle = random(TWO_PI);
    velocity = new PVector(cos(angle), sin(angle));

    position = new PVector(x, y);
    r = 2.0;
    maxspeed = 2;
    maxforce = 0.03;
  }

  void run(ArrayList<Boid> boids) {
    flock(boids);
    update();
    borders();
    render();
  }

  void applyForce(PVector force) {
    // We could add mass here if we want A = F / M
    acceleration.add(force);
  }

  // We accumulate a new acceleration each time based on three rules
  void flock(ArrayList<Boid> boids) {
    PVector sep = separate(boids);   // Separation
    PVector ali = align(boids);      // Alignment
    PVector coh = cohesion(boids);   // Cohesion
    // Arbitrarily weight these forces
    sep.mult(1.5);
    ali.mult(1.0);
    coh.mult(1.0);
    // Add the force vectors to acceleration
    applyForce(sep);
    applyForce(ali);
    applyForce(coh);
  }

  // Method to update position
  void update() {
    // Update velocity
    velocity.add(acceleration);
    // Limit speed
    velocity.limit(maxspeed);
    position.add(velocity);
    // Reset accelertion to 0 each cycle
    acceleration.mult(0);
  }

  // A method that calculates and applies a steering force towards a target
  // STEER = DESIRED MINUS VELOCITY
  PVector seek(PVector target) {
    PVector desired = PVector.sub(target, position);  // A vector pointing from the position to the target
    // Scale to maximum speed
    desired.normalize();
    desired.mult(maxspeed);

    // Above two lines of code below could be condensed with new PVector setMag() method
    // Not using this method until Processing.js catches up
    // desired.setMag(maxspeed);

    // Steering = Desired minus Velocity
    PVector steer = PVector.sub(desired, velocity);
    steer.limit(maxforce);  // Limit to maximum steering force
    return steer;
  }

  void render() {
    // Draw a triangle rotated in the direction of velocity
    float theta = velocity.heading2D() + radians(90);
    // heading2D() above is now heading() but leaving old syntax until Processing.js catches up
    
    fill(200, 100);
    stroke(255);
    pushMatrix();
    translate(position.x, position.y);
    rotate(theta);
    beginShape(TRIANGLES);
    vertex(0, -r*2);
    vertex(-r, r*2);
    vertex(r, r*2);
    endShape();
    popMatrix();
  }

  // Wraparound
  void borders() {
    if (position.x < -r) position.x = width+r;
    if (position.y < -r) position.y = height+r;
    if (position.x > width+r) position.x = -r;
    if (position.y > height+r) position.y = -r;
  }

  // Separation
  // Method checks for nearby boids and steers away
  PVector separate (ArrayList<Boid> boids) {
    float desiredseparation = 25.0f;
    PVector steer = new PVector(0, 0, 0);
    int count = 0;
    // For every boid in the system, check if it's too close
    for (Boid other : boids) {
      float d = PVector.dist(position, other.position);
      // If the distance is greater than 0 and less than an arbitrary amount (0 when you are yourself)
      if ((d > 0) && (d < desiredseparation)) {
        // Calculate vector pointing away from neighbor
        PVector diff = PVector.sub(position, other.position);
        diff.normalize();
        diff.div(d);        // Weight by distance
        steer.add(diff);
        count++;            // Keep track of how many
      }
    }
    // Average -- divide by how many
    if (count > 0) {
      steer.div((float)count);
    }

    // As long as the vector is greater than 0
    if (steer.mag() > 0) {
      // First two lines of code below could be condensed with new PVector setMag() method
      // Not using this method until Processing.js catches up
      // steer.setMag(maxspeed);

      // Implement Reynolds: Steering = Desired - Velocity
      steer.normalize();
      steer.mult(maxspeed);
      steer.sub(velocity);
      steer.limit(maxforce);
    }
    return steer;
  }

  // Alignment
  // For every nearby boid in the system, calculate the average velocity
  PVector align (ArrayList<Boid> boids) {
    float neighbordist = 50;
    PVector sum = new PVector(0, 0);
    int count = 0;
    for (Boid other : boids) {
      float d = PVector.dist(position, other.position);
      if ((d > 0) && (d < neighbordist)) {
        sum.add(other.velocity);
        count++;
      }
    }
    if (count > 0) {
      sum.div((float)count);
      // First two lines of code below could be condensed with new PVector setMag() method
      // Not using this method until Processing.js catches up
      // sum.setMag(maxspeed);

      // Implement Reynolds: Steering = Desired - Velocity
      sum.normalize();
      sum.mult(maxspeed);
      PVector steer = PVector.sub(sum, velocity);
      steer.limit(maxforce);
      return steer;
    } 
    else {
      return new PVector(0, 0);
    }
  }

  // Cohesion
  // For the average position (i.e. center) of all nearby boids, calculate steering vector towards that position
  PVector cohesion (ArrayList<Boid> boids) {
    float neighbordist = 50;
    PVector sum = new PVector(0, 0);   // Start with empty vector to accumulate all positions
    int count = 0;
    for (Boid other : boids) {
      float d = PVector.dist(position, other.position);
      if ((d > 0) && (d < neighbordist)) {
        sum.add(other.position); // Add position
        count++;
      }
    }
    if (count > 0) {
      sum.div(count);
      return seek(sum);  // Steer towards the position
    } 
    else {
      return new PVector(0, 0);
    }
  }
}

Functions Used

popMatrix()

Pops the current transformation matrix off the matrix stack

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rotate()

Rotates the amount specified by the angle parameter

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radians()

Converts a degree measurement to its corresponding value in radians

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mag()

Calculates the magnitude (or length) of a vector

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fill()

Sets the color used to fill shapes

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size()

Defines the dimension of the display window width and height in units of pixels

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cos()

Calculates the cosine of an angle

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random()

Generates random numbers

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stroke()

Sets the color used to draw lines and borders around shapes

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vertex()

All shapes are constructed by connecting a series of vertices

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sin()

Calculates the sine of an angle

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beginShape()

Using the beginShape() and endShape() functions allow creating more complex forms

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setup()

The setup() function is run once, when the program starts

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translate()

Specifies an amount to displace objects within the display window

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draw()

Called directly after setup(), the draw() function continuously executes the lines of code contained inside its block until the program is stopped or noLoop() is called

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mousePressed()

The mousePressed() function is called once after every time a mouse button is pressed

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endShape()

The endShape() function is the companion to beginShape() and may only be called after beginShape()

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background()

The background() function sets the color used for the background of the Processing window

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pushMatrix()

Pushes the current transformation matrix onto the matrix stack

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PVector

A class to describe a two or three dimensional vector, specifically a Euclidean (also known as geometric) vector

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dist()

Calculates the distance between two points

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