Ah, I misunderstood. I thought your score was the overlay, not the colored rectangles. Nonetheless, I think that something you are doing is changing the size of the area into which the score is being painted and that is resulting in the aliasing effect. It may not be something you yourself are doing in code, but a resize / relayout that is occurring behind the scenes when you display the semi-transparent overlay.

Try installing an event filter so you can watch the paint events on the QLabel so you can see if the size is changing. I'd use qDebug() rather than breakpoints so you don't interrupt the painting.