docs: public PointTrack addTrack( double x , double y ) {
  		if( !input.isInBounds((int)x,(int)y))
  			return null;
  
  		// grow the number of tracks if needed
  		if( unused.isEmpty() )
  			addTrackToUnused();
  
  		// TODO make sure the feature is inside the image
  
  		PyramidKltFeature t = unused.remove(unused.size() - 1);
  		t.setPosition((float)x,(float)y);
  		tracker.setDescription(t);
  
  		PointTrack p = (PointTrack)t.cookie;
  		p.set(x,y);
  
  		if( checkValidSpawn(p) ) {
  			active.add(t);
  			return p;
  		}
  
  		return null;
  	}