def phi_z(alpha)
      # If the ray is orthogonal on the z axis, the evaluation will yield NaN, and
      # we return the index k which corresponds to the source's z position instead.
      (((@p2.z - @p1.z == 0 ? @p1.z : pz(alpha)) - bz) / @vs.delta_z).floor
    end
def phi_z(alpha)
      # If the ray is orthogonal on the z axis, the evaluation will yield NaN, and
      # we return the index k which corresponds to the source's z position instead.
      (((@p2.z - @p1.z == 0 ? @p1.z : pz(alpha)) - bz) / @vs.delta_z).floor
    end
Calculates a voxel plane z index (k) from an alpha value.

 @param [Float] alpha a fraction of the ray's movement between source and target
 @return [Integer] the calculated voxel plane index (k)