	var velocity	= new THREE.Vector3(0, 1, 0)
		velocity.x	+= (Math.random()-0.5)*0.5
		velocity.y	+= (Math.random()-0.5)*0.1
		velocity.z	+= (Math.random()-0.5)*0.5
		velocity.setLength(3 + (Math.random()-0.5)*0.5)
		// init opacity
		var age2Opacity	= createTweenMidi(maxAge, 0.05*maxAge, 0.4*maxAge)
		material.opacity= age2Opacity(0)
		
		var birthDate	= Date.now()/1000
		var callback	= loop.hook(function(delta, now){
			var age	= Date.now()/1000 - birthDate
			if( age >= maxAge ){
				sprite.parent.remove(s