BSP index implementation

Dear list!
Please, could someone explain to me how the BSP index of QGraphicsScene works?
Specifically, how is it possible that
items
return a list sorted by zvalue? Is it a property of BSP index or are the items explicitly "zsorted" after a the index narrows down the possible candidates?Also, if BSP is used with triangles, the slitting plate of a triangle might cut another triangle into halves. QGraphicsScene does not operate over triangles but, I presume, over minimum bounding rectangles. Where does the splitting plane cut through? Is it axisaligned?
Thanks a lot in advance!

Hi,
That's the kind of question you should rather ask on the "interest mailing list":http://lists.qtproject.org/mailman/listinfo/interest You'll find there Qt's developers/maintainers (this forum is more user oriented)

Thanks for the hint, I just tried that.

By the way the QGraphicsScene is 2D so the z value is just a way to manipulate the draw order.

Yes, I know and that's exactly my point. Where/when are them items zsorted? Are they always kept zsorted? If I change a zvalue, is the whole BSP rebuilt?