QGraphicsView, QGraphicsItem, QGraphicsScene

[wysota]

I always tell to my students that the scene is the world, the view is the window they're looking through at the world and the items are things that are part of the world that one sees through the window. The world is self-sufficient (you don't need to have any windows for the world to be there and be useful) and objects in the world form hierarchies where children are positioned in a "local world" of their parent -- like people on Earth, Earth in the Solar System, Solar System in the Milky Way, Milky Way in the local cluster of galaxies, etc. And people don't care that Milky Way rotates around the centre of the local cluster, they use local coordinates relative to the Earth exactly the same way as items in the scene don't care about surroundings of its parent and use local coordinates relative to their parent.

[alitoh]

Thank you for your answers!

To extend on this issue a little bit more.

Are the QGraphicsScene coordinates relative to the Dlg window? That is; the single pixel left-top corner below the dialog bar, 0,0, is also the Scene's 0,0?

[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.