public static void merge(
     final LayerVisibility v1,
     final LayerVisibility v2,
     final TShortObjectHashMap<short[]> layerByGroups) {
   for (int i = 0; i < v1.size(); ++i) {
     final short[] layers = v1.getQuickValue(i);
     if (Arrays.binarySearch(layers, (short) 0) >= 0) {
       final short layer = v1.getQuickKey(i);
       final TShortHashSet set = new TShortHashSet(layerByGroups.get(layer));
       set.addAll(v2.getVisibleFromOutdoor());
       final short[] added = set.toArray();
       Arrays.sort(added);
       layerByGroups.put(layer, added);
     }
   }
 }