code: function involuteXbez(t)
    {
      // map t (0 <= t <= 1) onto x (where -1 <= x <= 1)
      var x = t*2-1;
      //map theta (where ts <= theta <= te) from x (-1 <=x <= 1)
      var theta = x*(te-ts)/2 + (ts + te)/2;
      return Rb*(Math.cos(theta)+theta*Math.sin(theta));
    }
docs: Equation of involute using the Bezier parameter t as variable