Geometric Distance Algorithm for Finding Initial Vertex Weights



This initial weight guessing algorithm uses mesh connectivity and geometric distance rather than Euclidean distance to decide which joints should influence a given vertex. This prevents a joint from influencing a vertex that, although very close in Euclidean distance, is not near to the joint in terms of the mesh. A classic example of this is in the knees of a biped character. The vertices at the inside of the knees may be very close to each other but you would never want a left knee joint to influence a right knee vertex, or vice versa.

The geometric distances between vertices is precomputed using Dijkstra's algorithm using each vertex as the source. This is an NxN matrix, for a mesh of N vertices. This is one way to compute All Pairs Shortest Paths for a graph, or in my case, a mesh.

So, you might ask, how do you compute distance from a vertex to a joint? The answer, cut the mesh with the joint coordinate plane and the store the "ring" of vertices in the mesh that approximate the plane-mesh intersections. This ring of vertices are at distance 0 from the joint, then use the precomputed All Pairs Shortest Paths matrix for the rest.

Below is a psuedocode outline of the algorithm:

/* Cut the mesh by each joint and get the ring of vertices. */
for each joint j in skeleton,
   Cut the mesh by the plane defined by j's coordinate system
   Store list of n vertices in the mesh that best approximate 
    the n plane-mesh intersections

/* Find initial influence set and weights for each vertex */
for each vertex v in mesh,
   Find the geometric distance d to each joint by finding the nearest vertex 
    in each plane-mesh approximation list

      if the current closest joint dj = min( D ) < Threshold, 
         Add joint j to v's influence set with weight = dj
         D = D - dj

   until we have selected the maximum number of influences

   Maximum-Distance = min( D )

   for each influence j,
      weightj = F( weightj / Maximum-Distance )

For the weight normalization function F I use a Cubic Hermite Spline. I tried a linear falloff function as well but this one produced qualitatively better results.

Cubic Hermite Spline


Luke Tokheim, <>, May 13, 2003