How to use QT.SetWindowOpacity in fvwm?

I want use Qt to change the window opacity. the Documents in QT said setWindowOpacity function on X11 need composite manager and specific _NET_WM_WINDOW_OPACITY atom supported by the wm.

Note : On X11 you need to have a composite manager running, 
and the X11 specific _NET_WM_WINDOW_OPACITY atom needs to be supported by the window manager 
you are using.

I use xcompmgr as the compositor, but don’t know if the fvwm supported the specific _NET_WM_WINDOW_OPACITY atom.

I can’t change the opacity of window, is it a problem with compositor or not supporting the specific _NET_WM_WINDOW_OPACITY atom ? and How do I know which window manager supports the specific _NET_WM_WINDOW_OPACITY atom?

thanks a lot!

I used picom insteading xcompmgr, and the opacity of window can be changed.

The picom sample file has many configuration options. It is good to start with this:

thanks, theSetWindowOpacity works well under the picom.