class Ring {
  float x, y;
  float diameter;
  boolean on = false;  // whether or not it's being shown

  // Turn ring on, starting to expand from (x0,y0)
  void start(float x0, float y0) 
  {
    x = x0; y = y0;
    on = true;
    diameter = 1;
  }

  void grow() 
  {
    if (on) {
      diameter += 0.5;
      if (diameter > 400) {  // too big -- stop it
        on = false;
      }
    }
  }

  // CBK variation
  void display()
  {
    if (on) {
      noFill();
      int d = round(diameter);
      int a = 200;
      // More opaque at larger diameter
      while (d > 0 && a > 0) {
        stroke(150, a);
        ellipse(x, y, d, d);
        d -= 10;
        a -= 20;
      }
    }
  }
}