private function isBackFace():Boolean
    {
     // see http://www.jurjans.lv/flash/shape.html
     var cax:Number = pointC.screenX - pointA.screenX;
     var cay:Number = pointC.screenY - pointA.screenY;
     
     var bcx:Number = pointB.screenX - pointC.screenX;
     var bcy:Number = pointB.screenY - pointC.screenY;
     
     return cax * bcy > cay * bcx;
    }