public static Segment intersectionTrianglePlan(Face f, double z) {
		Point a = intersectionSegmentPlan(f.p1, f.p2, z);
		Point b = intersectionSegmentPlan(f.p2, f.p3, z);
		Point c = intersectionSegmentPlan(f.p3, f.p1, z);

		if (b != null && c != null) {
			return new Segment(b, c);
		} else if (a != null && c != null) {
			return new Segment(a, c);
		} else if (a != null && b != null) {
			return new Segment(a, b);
		}
		return null;
	}