The current QVTKQuickItem implementation uses the Qt Quick integration approach of painting the VTK scene to the screen before the QSG[1]. This has a few significant drawbacks:
- You must remember that Qt Quick stacking order does not apply to QVTKQuickItem instances in your Qt Quick application. So doing something as simple as drawing a solid black rectangle for the application background no longer works.
- No item properties (other than x, y, width, height) can be applied or animated via Qt Quick. So fading out a view port during layout changes or animating a left-right or top-bottom flip of the viewport are not possible.
- The QSG optimizations to repaint items only when necessary cannot be applied to QVTKQuickItem instances. So if you have, say, a pulsing animation that is not superimposed over any VTK viewport, the QSG will still re-render all the VTK viewports for every frame of the 60 FPS pulsing animation.
[1] http://qt-project.org/doc/qt-5/qtquick-visualcanvas-scenegraph.html