painter->drawArc(RectBuffForPos, 45*16,180*16);

the actual startAngle is more than 45°，see the first picture（the black line draw by hand）。

painter->drawArc(RectBuffForPos, 45*16,270*16);

see the second picture，the origin coordinates is on the left and no in center。only square is in center。

the test app is drawArcTest.rar file.