Iterative thinning skeletonization methods can provide a distance in voxels from each tree node to the surface of the object. This distance is the D6 metric, obtained by counting the number of voxels as they are removed in the minimal 6-connected path from the surface to the medial axis.
Methods for iterative thinning can determine the distance between each tree node and the surface of an object using a metric called the D6.  This metric is derived by counting the number of voxels that are removed during the process of creating a skeleton, which follows the shortest path from the surface to the medial axis.