class Particle {
  float ox, oy;  // original position
  float x, y;    // current position
  float vx, vy;  // velocity
  float t;       // transparency
  float dt = 3;  // transparency step
  float g = 0.1; // gravity
  float r = 3;   // radius
  
  Particle(float x0, float y0)
  {
    ox = x0; oy = y0;
    initialize();
  }

  // Put at original position, fully opaque, with random velocity
  void initialize()
  {
    x = ox; y = oy;
    // More vertical than horizontal
    vx = random(-3,3); vy = random(-5,-1);
    t = 255;
  }
  
  void draw()
  {
    fill(255,t);
    ellipse(x,y,2*r,2*r);
  }
  
  void update()
  {
    t -= dt;    // decay the transparency
    // Usual gravity stuff
    vy += g;
    x += vx;
    y += vy;
    // When exit screen or totally transparent, re-initialize
    if (t < 0 || x>width+r || x<r || y>height+r || y<r) 
      initialize();
  }
}