// Daniel Shiffman

// http://natureofcode.com

​

// A circular particle

​

// Constructor

class Particle {

  constructor(x, y) {

    this.r = 8;

​

    // Define a body

    let bd = new box2d.b2BodyDef();

    bd.type = box2d.b2BodyType.b2_dynamicBody;

    bd.position = scaleToWorld(x, y);

​

    // Define a fixture

    let fd = new box2d.b2FixtureDef();

    // Fixture holds shape

    fd.shape = new box2d.b2CircleShape();

    fd.shape.m_radius = scaleToWorld(this.r);

​

    // Some physics

    fd.density = 1.0;

    fd.friction = 0.1;

    fd.restitution = 0.3;

​

    // Create the body

    this.body = world.CreateBody(bd);

    // Attach the fixture

    this.body.CreateFixture(fd);

​

    // Some additional stuff

    this.body.SetLinearVelocity(new box2d.b2Vec2(random(-5, 5), random(2, 5)));

    this.body.SetAngularVelocity(random(-5, 5));

  }

​

  // This function removes the particle from the box2d world

  killBody() {

    world.DestroyBody(this.body);

  }

​

  // Is the particle ready for deletion?

  done() {

    // Let's find the screen position of the particle

    let pos = scaleToPixels(this.body.GetPosition());

    // Is it off the bottom of the screen?

    if (pos.y > height + this.r * 2) {

      this.killBody();

      return true;

    }

    return false;

  }

​

  // Drawing the box

  display() {

    // Get the body's position

    let pos = scaleToPixels(this.body.GetPosition());

    // Get its angle of rotation

    let a = this.body.GetAngleRadians();

​

    // Draw it!

    rectMode(CENTER);

    push();

    translate(pos.x, pos.y);

    rotate(a);

    fill(127);

    stroke(200);

    strokeWeight(2);

    ellipse(0, 0, this.r * 2, this.r * 2);

    // Let's add a line so we can see the rotation

    line(0, 0, this.r, 0);

    pop();

  }

}