@wrosecrans
I know that I need them. But don‘t know how to apply them on the scene and keep everything thats on that. I thought that the rect itself has the points in it using the render function of the QGraphicsScene is enough.

What I do: I set the DragMode of the QGraphicsView to rubberband so the user can select some part pf the scene by simply dragging. The returned rect is now sent to the view to get the underlying image of the scene.

Yes, I've seen it before. But the thing is, I'd like to 1) crop a fragment of it later, 2) save it as an image.