Custom QSGGeometryNode Scene blending issue

[che1404]

Hi all!
We have a custom QQuickItem which in turn creates a custom QSGGeometryNode. We reimplement the QSGGeometryNode's preprocess() method in order to draw our scene to a FrameBufferObject with a texture attached. This OpenGL texture ID is shared with the QSGGeometryNode's material. This way, we can render our custom OpenGL scene into a QML item. So far, so good.

However, we want to use alpha blending on some parts of our scene, in order to let the user see the QML items behind our custom item. Something similar to what Item::opacity value is doing, but only for a specific area of our Item. It's like having a hole inside our Item that allows the user to see what's behind.

I've been trying to find out what Qt is doing in order to change the opacity of a QML item, with no luck.

Any idea on how can we achieve this effect, with our Item/Node configuration?
Thanks in advance,
Robert.