function p_alphafade(PARTICLE *p)
{
    p.alpha -= p.skill_a*time_step;
    if (p.alpha <= 0) p.lifespan = 0;
}

function p_trace(PARTICLE *p)
{
   set(p, BRIGHT|TRANSLUCENT|BEAM);
   p.size = 2;
   p.skill_a = 1; // fade factor
   p.event = p_alphafade;
}

// entity that runs in circles and leaves a vapor trail
action tracer()
{
   var dist = 0,radius = 10,sign = 1;
   VECTOR last_pos;
   while(1) 
   {
      vec_set(last_pos,my.x);
      dist += 30*time_step;
      radius += sign*random(3)*time_step; // change radius randomly
      if (radius > 150) sign = -1;
      else if (radius < 30) sign = 1;
      my.x = radius*sin(dist);
      my.y = radius*cos(dist);
      effect(p_trace,1,my.x,vec_sub(last_pos,my.x));
      wait(1);
   }
}

function main()
{
  vec_set(sky_color,vector(50,1,1)); // dark blue
  level_load(NULL);
  video_window(NULL,NULL,0,"Vapor trail demo");
  vec_set(camera.x,vector(-250,0,50));
  vec_set(camera.pan,vector(0,-15,0));

  ent_create(NULL,vector(0,0,0),tracer);  
}