Qt Overlay over GStreamer

I've a question considering the current setup:

• Yocto Linux on iMX6
• Neither a window-, nor a display-manager
• A fully functional Qt Application, tested on Debian 9

The application consists of 2 main elements:

• A GStreamer part, with a imxg2dvideosink
• A semi-transparent Qt Overlay, which should be displayed over the stream

The question:

How can I accomplish to display the overlay over the stream, while having both parts on fullscreen (filling the whole screen)? Possible solutions:

• /dev/fb1 as an overlay to /dev/fb0 (How to split a single application to two fb's ?)
• Use a display-manager ?
• Use a window-manager ?
• linuxfb instead of eglfs ?

My current (not working) solution:

• Using -platform eglfs
• The application will first start GStreamer, and afterwards show the overlay

I've found the solution myself. Shared below:

1) Run Qt Application on /dev/fb1:

• export QT_QPA_EGLFS_FB=/dev/fb1 (Specify /dev/fb1 as eglfs framebuffer)
• echo 0 > /sys/class/graphics/fb1/blank (Unblank framebuffer)
• fbset -fb /dev/fb1 --geometry <your geometry here> (Set framebuffer geometry)
• ./YourApplication -platform eglfs (Run application)

Use a Color Key if you want full opacity while having fully transparent parts of your overlay.

2) Run GStreamer on /dev/fb0:

• gst-launch-1.0 videotestsrc ! imxg2dvideosink framebuffer=/dev/fb0

This is the solution for eglfs. Other possibilities are linuxfb.