Any thoughts about experience with integration OpenGL with QtQuick/Widgets are welcome. Maybe I'll have to check it myself how it behaves. I've read some materials about it (mostly from KDAB) but they usually aim simple case where synced data is really small eg. camera info + a few additional bytes of data.
@Devopia53 I tested on Qt 5.8/Win7 today. The same incorrect result. But today i discovered the difference in overlaping behavior on my two machines. On the machine with video card fully-transparent widget has black color on white opengl context :D, where as other machine with integrated video card has no color(as should). So the gui transparency on opengl context is very platform dependent. Solution is to render widget by opengl directly but this way is very inconvenient. I think troll-tech must solve this problem in future.
Assuming you have an OpenGL context and you can draw/create objects in it you would make that context current, then create a QOpenGLPaintDevice and use it to render() the widget into that. Please note that a widgets require a QApplication instance and a running Qt's event loop, so you would need to ensure you have that.
As for DirectX Qt doesn't have any direct support for it, but you could probably grab() a widget into a QPixmap, transfer the pixels to a DirectX texture and draw a quad with it.
If you feel adventurous you could also implement your own QPaintEngine, then subclass QPaintDevice to return that engine and use it like with the OpenGL version.