code: public Complexd toComplex() {
          final double w2 = w * w;
          return new Complexd(2 * w2 - 1, 2 * w * (double) Math.sqrt(1 - w2));
      }
docs: Converts this quaternion to a complex by extracting the rotation around
  the axis and returning it as a rotation in the plane perpendicular to the
  rotation axis.
  
  @return The rotation without the axis as a complex