code: def _opdist_low(av, bv, cv, deriv):
      
      a = Vector3(9, deriv, av, (0, 1, 2))
      b = Vector3(9, deriv, bv, (3, 4, 5))
      c = Vector3(9, deriv, cv, (6, 7, 8))
      n  = cross(a, b)
      n /= n.norm()
      dist = dot(c, n)
      return dist.results()
docs: Similar to opdist, but with relative vectors