QGraphicsView, QGraphicsItem, QGraphicsScene

[wysota]

Again, QGraphicsScene is the world. The widget (view) is a window in a house. Coordinates of the world are not relative to the window in the house because the house may not even exist. Of course it is possible to map coordinates of each place in the world relative to the glass in the window in the house provided the window in the house exists. This is called a projection and its properties change if you come closer or move away from the window surface. This doesn't change the world (scene) in any way although if you change your position, you see a different part of the world. So to answer your question the top-left corner of the window is not the top-left corner of the world.