code: private static <P extends GISPrimitive, N extends AbstractGISTreeSetNode<P, N>>
  		Point2d computeCutPoint(IcosepQuadTreeZone region, N parent) {
  		final double w = parent.nodeWidth / 4.;
  		final double h = parent.nodeHeight / 4.;
  		final double x;
  		final double y;
  		switch (region) {
  		case SOUTH_WEST:
  			x = parent.verticalSplit - w;
  			y = parent.horizontalSplit - h;
  			break;
  		case SOUTH_EAST:
  			x = parent.verticalSplit + w;
  			y = parent.horizontalSplit - h;
  			break;
  		case NORTH_WEST:
  			x = parent.verticalSplit - w;
  			y = parent.horizontalSplit + h;
  			break;
  		case NORTH_EAST:
  			x = parent.verticalSplit + w;
  			y = parent.horizontalSplit + h;
  			break;
  		case ICOSEP:
  		default:
  			return null;
  		}
  		return new Point2d(x, y);
  	}
docs: Computes the cut planes' position of the given region.
  
  @param region is the id of the region for which the cut plane position must be computed
  @param parent is the parent node.
  @return the cut planes' position of the region, or <code>null</code> if
  the cut planes' position could not be computed (because the region is the icosep region
  for instance).