对于 QGraphicsScene，在 QGraphicsItem 上使用 setZValue 会导致严重减速 [英] For QGraphicsScene using setZValue on a QGraphicsItem causes major slowdown
问题描述
我正在使用 Qt(Mac，4.7 版)通过 QGraphicsView 和 QGraphicsScene 渲染相当大的场景(来自 Open Street Maps 的地图数据).一切都很好，直到我尝试为场景中的项目设置 Z 值.我尝试了两种不同的方法来做到这一点.一方面，我只是这样做:
I'm using Qt (Mac, version 4.7) to rendering a rather large scene (map data from Open Street Maps) with QGraphicsView and QGraphicsScene. Everything works great until I try to set the Z value for the items in the scene. I've tried two separate approaches to do this. In one, I just do:
QGraphicsPathItem *item = scene->addPath(path, pen);
item->setZValue(z);
另一个我创建了自己的 QGraphicsItem 子类，但遇到了完全相同的问题.
and the other I create my own QGraphicsItem subclass, but get the exact same problem.
速度变慢的原因似乎是在 Qt 方面.一旦我设置了 z 值，场景生成需要很长时间(在任何显示之前需要几分钟，我得到旋转的死亡沙滩球)，但是一旦生成，速度就会恢复正常.此外，当我尝试关闭应用程序时，它会在关闭前挂起几分钟.如果我不考虑 z 值，我不会看到任何这些问题，并且我添加了调试代码以验证它没有出现在我自己的代码中.
The cause of the slow down appears to be on the Qt side of things. It takes a very long time for the scene to generate once I set the z-value (it takes a few minutes before anything displays, and I get the spinning beach ball of death), but once it's generated, the speed is back to normal. Additionally, when I try to close the application it hangs for a few minutes before closing. I don't see any of these issues if I leave the z-value alone, and I've added debugging code to verify it's not occurring in my own code.
不幸的是，我需要设置 z 值才能正确渲染街道(例如，道路周围的轮廓、使高速公路比穿过街道的顺序更高等).
Unfortunately, I need to set the z-value in order to render streets correctly (e.g. outlines around roads, make highways a higher order than through streets, etc.).
非常感谢任何帮助！
推荐答案
您可以尝试设置 bspTreeDepth 到一个固定值.它控制场景中项目的位置.使用大量现有项目更改深度的成本可能非常高.
You may try setting bspTreeDepth to a fixed value. It controls how an item in the scene is located. Changing the depth with a large number of existing items can be very costly.
另一个优化候选是itemIndexMethod.将其设置为 noIndex 实际上可能会提高性能.
Another candidate for optimization is itemIndexMethod. Setting it to noIndex may actually increase the performance.
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