Fig. 3.2.

One-ring-strip data structure: in this figure, T_i is a tetrahedron, x_i, y_i and z_i represent the coordinates of the ith vertex, f_i is the inverse of speed on a vertex. Φ_i denotes the value of the solution at the ith vertex. I_i in STRIP represents the data structure for the one-ring strip of the ith vertex each of which has q indices pointing (shown as arrows) to the value array.