//  Self-propelled particles 

//  

//  Modeled on https://www.nature.com/articles/srep37969

//

​

let particles = [];

let torus = true;

let v = 5.0;  //  Constant velocity, pixels/step

​

function setup() {

  createCanvas(windowWidth, windowHeight);

  for (let i = 0; i < 2000; i++) {

    particles.push(new Particle());

  }

}

​

function draw() {

  background(220);

  for (let particle of particles) {

    particle.update();

    particle.show();

  }

}

​

function left(pos1, vel1, pos2) {

  let relPos = p5.Vector.sub(pos2, pos1);

  let crossProduct = relPos.x * vel1.y - relPos.y * vel1.x;

  return crossProduct > 0;

}

​

function rotateVelocity(vel, angle) {

  let radianAngle = radians(angle);

  let newVel = createVector(

    vel.x * cos(radianAngle) - vel.y * sin(radianAngle),

    vel.x * sin(radianAngle) + vel.y * cos(radianAngle)

  );

  return newVel;

}

​

class Particle {

  constructor() {

    this.pos = createVector(random(width), random(height));

    this.vel = createVector(0, v);

    this.vel = rotateVelocity(this.vel, random(360.0) );

  }

​

  update() {

    let leftCount = 0;

    let rightCount = 0;

    let alpha = 180.0;    //  constant turn 

    let beta = 17.0;     //  strength of neighor influence

    let radius = 50;    //  radius of detection (pixels)

    for (let other of particles) {

      if ((other !== this) && (p5.Vector.sub(other.pos, this.pos).mag() < radius)) {

        if (left(this.pos, this.vel, other.pos)) leftCount++;

        else rightCount++;

      }

    }

    if (rightCount > leftCount) {

      this.vel = rotateVelocity(this.vel, alpha + beta*(rightCount + leftCount));  

    } else {

      this.vel = rotateVelocity(this.vel, alpha - beta*(rightCount + leftCount));

    }

    this.pos.add(this.vel);

    if (torus) {

      this.pos.x = (this.pos.x + width) % width;

      this.pos.y = (this.pos.y + height) % height;

    } else {

      // Bounce off edges

      if (this.pos.x < 0 || this.pos.x > width) {

        this.vel.x *= -1;

      }

      if (this.pos.y < 0 || this.pos.y > height) {

        this.vel.y *= -1;

      }  

    }

  }

​

  show() {

    stroke(0);

    strokeWeight(2);

    fill(175);

    ellipse(this.pos.x, this.pos.y, 6, 6);

  }

}