private static void renderHull(PtEntry pt) {
    Inf inf = Inf.create();
    int count = 30;
    PtEntry ptStart = pt;
    do {
      inf.update();
      if (pt.prev(true) != null) {
        boolean valley = false;

        valley =
            (pt.prev(true).source() == pt.source() && pt.prev(true).orig().next(true) != pt.orig());

        V.pushColor(MyColor.cBLUE);
        V.pushStroke(valley ? STRK_RUBBERBAND : STRK_THICK);
        V.drawLine(pt.prev(true), pt);
        V.pop(2);
      }
      V.pushColor(MyColor.cDARKGREEN);
      V.mark(pt, MARK_DISC, .6);
      V.pop();
      if (Editor.withLabels(true)) {
        StringBuilder sb = new StringBuilder();
        sb.append(" #" + pt.id());
        if (pt.source() != null) sb.append(" <" + pt.source() + ">");

        V.pushScale(.6);
        V.draw(sb.toString(), MyMath.ptOnCircle(pt, MyMath.radians(30), 3), TX_FRAME | TX_BGND);
        V.pop();
      }
      pt = pt.next(true);
      if (count-- == 0) V.draw("too many points rendering!", 50, 50, TX_CLAMP);
    } while (pt != ptStart && count > 0);
  }
  /**
   * Calculate possible hull of a pair of polygons
   *
   * @param a
   * @param b
   * @return possible hull
   */
  private static EdPolygon calcPHull(EdPolygon aPoly, EdPolygon bPoly) {
    final boolean db = true;

    if (db && T.update())
      T.msg(
          "calc possible hull of two polygons"
              + T.show(aPoly, MyColor.cBLUE, STRK_THICK, -1)
              + T.show(bPoly, MyColor.cDARKGREEN, STRK_THICK, -1));
    PtEntry a = PtEntry.buildFrom(aPoly);
    PtEntry b = PtEntry.buildFrom(bPoly);

    PtEntry aHull = PtEntry.convexHull(a);
    PtEntry bHull = PtEntry.convexHull(b);

    inf = Inf.create();

    if (db && T.update()) {
      T.msg(
          "convex hulls"
              + T.show(aHull.toPolygon(), MyColor.cBLUE, STRK_THICK, -1)
              + T.show(bHull.toPolygon(), MyColor.cDARKGREEN, STRK_THICK, -1));
    }

    PtEntry ph = hullOfPolygons(a, b, aHull, bHull);
    traceHullPt = ph;

    insertValleys(ph, a.source(), b.source());

    expandHull(ph, aHull, bHull, true);
    expandHull(ph, aHull, bHull, false);

    return ph.toPolygon();
  }