What anti-aliasing algorithm does QPainter take to draw a Polygon?

[MartinChan 0]

I am trying to imitate the QPainter way to do anti-aliasing , and when I draw a circle ,and it's edge was like this.
QPainter( using drawPolygon with anti-aliasing ):

My Gaussian:

My bilateral (after Gaussian):

So I still cant get a Qt's style anti-aliasing ,can anybody tell what algorithm does QPainter take to do Anti-aliasing?