Repaint widget while resizing

Into a QMainWindow I have a QWidget that draw a QImage (to simulate a video). But when I resize the window without releasing the mouse button (after move) update() or repaint() haven’t effects on the QWidget. Yet, update() or repaint() method is called by a slot. How to repaint the widget while resizing ? I’m working on Qt Creator 2.6.1 with Qt 5.0.0 on OS X 10.8.2.