QtQuick 2.0 scene on top of Direct3D scene

janp

Hi everyone,

I have been trying to come up with a solution for having a QtQuick 2.0 scene together with a Direct3D scene for quite a while, but wasn't very successful. My goal is to have a Direct3D engine running at reasonable speed (60 FPS?) together with QML UI on top. Both things run just fine at 150-200 FPS on their own. But when forced to cooperate together within one window, everything just goes bananas. I have investigated several approaches, but none of them seems to be sufficient enough:

Solution A: Rendering Direct3D scene into a texture, visualizing with QImage & QQuickPaintedItem

this solution works quite well and it seems to be the preferred one according to other people on the web. However it is TERRIBLY slow. I wasn't able to have more than 18-20 FPS in full HD. The bottleneck was clearly in the texture transfer chain from GPU (D3D) to CPU (QImage) and back to GPU (QML renderer) each frame. Especially the CPU->GPU processing on the QML side was way too slow!

Solution B: Rendering QtQuick scene into a FBO, then using Direct3D texture

this is basically the previous solution other way round. The speed is a little bit better when the UI does not require an update. Once it starts animating, everything drops down to 18-20 FPS again. QOpenGLFramebufferObject::toImage() obviously takes its time. Implementing texture/FBO double buffering on both sides to reduce stalls does not really help.

Solution C: QQuickView with enabled transparency on top of QWidget with Direct3D scene

was not lucky with this approach either. It seems like the transparency works only when QQuickView is in its own window. Once I put it on top of my D3D QWidget within the same window, it immediately stopped working and became fully opaque. Someone was trying to do something similar there as well: http://qt-project.org/forums/viewthread/5484 , but I had no luck with that solution at all. Maybe keeping two completely separated windows (main D3D window + frameless transparent QML window) on top of each other all time would do the trick, but that just sounds silly.

Solution X: Modify ANGLE library and try to extract & share D3D device context with my Direct3D renderer

haven't tried this yet, avoiding any library modifications as long as possible. Would that even be a sensible option?

My obvious questions here are: Am I doing something wrong? What is the preferred solution? A, B, C, X or maybe something totally different? Can someone point me to the right direction?

TL;DR: What is the fastest way to render QML scene on top of Direct3D scene?

Thank you for your time and help!
Jan