code: function approximate_unit_arc(theta1, delta_theta) {
    var alpha = 4/3 * Math.tan(delta_theta/4);
  
    var x1 = Math.cos(theta1);
    var y1 = Math.sin(theta1);
    var x2 = Math.cos(theta1 + delta_theta);
    var y2 = Math.sin(theta1 + delta_theta);
  
    return [ x1, y1, x1 - y1*alpha, y1 + x1*alpha, x2 + y2*alpha, y2 - x2*alpha, x2, y2 ];
  }
docs: Approximate one unit arc segment with bézier curves, see http://math.stackexchange.com/questions/873224