def az(k)
      val = (coord_z(k) - @p1.z) / (@p2.z - @p1.z)
      val.nan? ? Float::INFINITY : val
    end
def az(k)
      val = (coord_z(k) - @p1.z) / (@p2.z - @p1.z)
      val.nan? ? Float::INFINITY : val
    end
Calculates the fraction of the ray's movement (in the z direction), between
 its source and destination, for the ray's travel to the given plane index k.

 @note This function may return values outside the interval <0, 1> for z
   indices that are outside the range of indices 'between' source and destination.
 @param [Integer] k a plane index
 @return [Float] a fraction of movement between source and target