class CannonBall extends Particle {

  color ballColor;
  float diameter;

  CannonBall() {    
    ballColor = color(random(255), random(255), random(255));
    diameter = 20 + random(70);
    minCollisionForce = diameter/2;
    maxCollisionForce = diameter;
    enableCollision(diameter/2 + 2);
  }

  void draw() {
    ellipseMode(CENTER_DIAMETER);
    push();
    translate(pos[0], pos[1], pos[2]);
    noStroke();
    fill(ballColor);
    ellipse(0,0,diameter,diameter);    
    pop();
    
    if (age > 200 && 
         (pos[0] < -diameter ||            // test for offscreen 
          pos[0] > width + diameter || 
          pos[1] > height + diameter) )
      die();
  }
}

PSystem ps;

void setup() {
  size(800, 500);
  ps = (PSystem)loadPlugin("PSystem");
  ps.setGravity(0.5);
  framerate(45);
}


void fireLeft() {
    Particle p = new CannonBall();
    p.pos[0] = 0;
    p.pos[1] = height;
    p.pos[2] = 0;
    
    ps.addParticle(p);
    
    p.velocity[0] = random(30);
    p.velocity[1] = random(-15) - 15;
}

void fireRight() {
    Particle p = new CannonBall();
    p.pos[0] = width;
    p.pos[1] = height;
    p.pos[2] = 0;
    
    ps.addParticle(p);
    
    p.velocity[0] = random(-30);
    p.velocity[1] = random(-15) - 15;
}

int skipLoops = 10;

void loop() {
  background(255);
  
  if (frame % skipLoops == 0) {
    fireLeft();
    fireRight();
  }
  
  ps.draw(); 
}