To build an octree for the current frame, a minimum cubic box
(cell) is the starting point to construct an octree. It includes all the
vertices of the 3D objects within the current frame. A motion
vector is associated with each corner of the cell. These eight
motion vectors approximate the motion of the enclosed vertices
using interpolation of the motion vectors over the space. Motion
vectors are obtained by using a Least Square Error estimation
method to estimate the motion of the enclosed vertices. If the
error of reconstructed motion is smaller than the specified
threshold, no splitting process is needed. Otherwise, the current
cell needs to be further subdivided into eight smaller cells and
each has eight new motion vectors. The splitting process
continues until the motion of the enclosed vertices can be
approximated within the specified threshold.