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);
}