How to implement OSD overlay in Wayland/Weston

[Guillermo Rodriguez]

Hi all,

This is a follow up to my earlier email "Let pointer events pass through".

I have an use case where there is one full screen application (I'll
call this the "main" application) and I need to show a OSD overlay on
top of it. The OSD overlay is actually based on GStreamer (think PiP)
and is managed by a different process.
The OSD overlay should not handle pointer events; these should pass
through to the main application, which knows how to handle them.

I am not sure how to approach this in Wayland/Weston. I have
considered the following:

1.
Use wl_surface::set_input_region so that pointer events are not
delivered to the OSD overlay, but instead pass through and are handled
by the main application.
This "almost" works, however when the main application receives the
event, Weston brings it on top, thus hiding the OSD overlay.
I searched for a way to specify that a given surface should always
stay "on top". This does not seem to be supported in Wayland.

2.
Setup things so that the OSD overlay is not an additional top level
surface, but a child surface owned by the main application.
This would mean having the main application create a child surface for
the OSD, and pass this surface to the OSD process.
In X this could be achieved by passing a Windows ID. In Wayland
apparently it is not possible to share surfaces between processes.

3.
Let the OSD overlay actually handle pointer events, but "send" them to
the main application.
In X this could be achieved with XSendEvent. In Wayland apparently it
is not possible to have one process send events to another process.

Not sure what else to try.

Any ideas? Either ways to make one of the above work, or new approaches...

TIA,

Guillermo Rodriguez