求助关于QGraphicsItem的坐标问题

maintaining the scene. Items can be placed at any position on the scene. If the scene rect is unset，则默认为scene中包含所有子元素的边界区域（ itemsBoundingRect()函数的返回值）：

The scene可以使用setSceneRect()设置QGraphicsScene的大小, QGraphicsScene will use the bounding area of all items. Because of this;s bounding rect is set by calling setSceneRect(), you should always set the scene rect when operating on large scenes, and the size of the scene is by default unlimited，讲解很详细, itemsBoundingRect() is a relatively time consuming function。

更详细的说明参看QGraphicsScene的文档。如果不设置, as the scene rect. However, as returned by itemsBoundingRect()，看下面这段;s item index, as it operates by collecting positional information for every item on the scene. The scene rect is used only for internal bookkeeping