class Particle {
  float ox, oy;  // original position
  float x, y;    // current position
  float vx, vy;  // velocity
  float t;       // tint
  float dt = 3;  // tint step
  PImage img;
  
  Particle(float x0, float y0, PImage img)
  {
    ox = x0; oy = y0; 
    this.img = img;  // distinguish parameter from field
    initialize();
  }

  // Put at original position, fully opaque, with random velocity
  void initialize()
  {
    x = ox; y = oy;
    t = 255;
    // Mostly vertical
    vx = (float)generator.nextGaussian()*0.3;
    vy = (float)generator.nextGaussian()*0.3 - 1;
  }
  
  void draw()
  {
    imageMode(CORNER);
    tint(255,t);
    image(img, x-img.width/2, y-img.height/2);
  }
  
  void update(float wind)
  {
    t -= dt;    // decay the transparency
    x += vx+wind;
    y += vy;
    // Re-initialize when decayed away
    if (t < 0) initialize();
  }
}