Overlapping brushes with different patterns

Hello, I am making a simple brushing/drawing interface with QGraphicsScene. The strokes are made with addEllipse with various brush sizes and styles, like Qt::Dense{1:7}Pattern. The problem is when a brush is drawn over already painted pixels with a different pattern. It seems like it is painting the union between the two patterns, making the area completely black (using a black colour of the brush) or some denser pattern. What I want is to replace the old pattern with the new brush. Any ideas on how I can do this?