var culling_distance = 4000;
action NULL_() 
{
 while(me)
 {
	if (vec_dist (my.x, camera.x) < culling_distance) // the player has come close to this entity?
	{
		reset(my, INVISIBLE); // then show it!
		while (my.alpha < 100) // run this loop until the entity becomes opaque again
		{
			my.alpha = minv(100, my.alpha + 15 * time_step); // increase my.alpha (15 = speed) and limit it to 100
			wait (1);
		}
		reset(my, TRANSLUCENT); // get rid of some nasty artifacts when the entity is completely visible
	}
	else // the player has moved away from this entity?
	{
		set(my, TRANSLUCENT);  // then set the "transparent" flag again
		while (my.alpha > 0) // run this loop until the entity becomes practically invisible (alpha = 0)
		{
			my.alpha = maxv(0, my.alpha - 15 * time_step); // decrease my.alpha (15 = speed) and limit it to 0
			wait (1);
		}
		set(my, INVISIBLE); // now hide the entity in order to increase the frame rate

	}
        wait(1);
 }
}