code: private double getDistance(TreeNode v, TreeNode w) {
  		double sizeOfNodes = getNodeSize(v) + getNodeSize(w);
  
  		double distance = sizeOfNodes / 2
  				+ configuration.getGapBetweenNodes(v, w);
  		return distance;
  	}
docs: The distance of two nodes is the distance of the centers of both noded.
  <p>
  I.e. the distance includes the gap between the nodes and half of the
  sizes of the nodes.
  
  @param v
  @param w
  @return the distance between node v and w