code: def __f2d(frac_coords, v):
          
          # frac_coords = frac_coords % 1
          return np.array([int(frac_coords[0] * v.shape[0]),
                           int(frac_coords[1] * v.shape[1]),
                           int(frac_coords[2] * v.shape[2])])
docs: Converts fractional coordinates to discrete coordinates with respect to
          the grid size of v