private List<Integer> getParticlesWithin(float x, float y, float r) {
		int n = model.getNumberOfParticles();
		if (n == 0)
			return null;
		List<Integer> list = new ArrayList<Integer>();
		Particle p = null;
		double dx, dy;
		for (int i = 0; i < n; i++) {
			p = model.getParticle(i);
			dx = p.getRx() - x;
			dy = p.getRy() - y;
			if (dx * dx + dy * dy < r * r) {
				list.add(i);
			}
		}
		return list;
	}