Foreign surface

[Pekka Paalanen]

On Thu, 26 Nov 2020 14:53:05 -0500
gherissi ayachi <gayachi at matrox.com> wrote:

> Hi,
> 
> Background: our library allow to grab video to a supplied windows 
> handle, When using X11 the user pass the XID to the lib

Hi,

just in case I'm not understanding the question right: you can't read
back window contents using Wayland. You need to make the application
give the content to your library and anything else your library needs
yourself.

> and we can paint to this Window and do some interaction (pointer), All 
> is done in the same process.
> 
> In Wayland, The user has to provide us with his wl_display and 
> wl_surface and we have to use  subsurface to do the paint right ?

Yes, that's one possibility, and the recommended one if the additional
content is (particularly hardware decoded) video. The catch is that you
need to carefully orchestrate all rendering and resizing with the
application. Even if you normally have the sub-surface in
desynchronized mode, you need to temporarily switch it to synchronized
mode during window resizing, so that the window does not "break into
parts" momentarily. This requires explicit API between the application
and your library, Wayland is not enough.

The other possibility is to not touch Wayland at all, and make the
application call into your library to draw the additional content as
part of drawing its own content. For video this is usually sub-optimal.

If your library does create new wl_surfaces, you also need to make sure
that the application can cope with Wayland events that deliver your
library's wl_surface to the application, e.g. in wl_pointer.enter.

Input events from Wayland perspective are no problem: your library
can bind to all wl_seats and create its own wl_pointers etc. and those
will deliver the same events as the application will get through its own
wl_pointer etc. The catch is the above: both the application and the
library need to understand which wl_surface is theirs and ignore input
events targetting wl_surfaces that are not theirs.

All the above assumes that your library is using the same wl_display
instance as the application. If they don't, they become two separate
Wayland clients, which is effectively the same as separate processes.

Btw. "foreign surface" is usually used to refer to cross-process
references to Wayland surface, e.g. by using
https://gitlab.freedesktop.org/wayland/wayland-protocols/-/blob/master/unstable/xdg-foreign/xdg-foreign-unstable-v2.xml .
That is not necessary if you can share the wl_display instance.


Thanks,
pq
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