Screenshot through x11 on Xwayland

[Zdeněk Žamberský]

On 18. 10. 21 23:49, Zdeněk Žamberský wrote:
> Hi,
>
> I experimented a bit with X11 approach. I made simple program [1], 
> which obtains
> pixel data for each top-level X11 window, combines them to single 
> buffer and then
> saves it to png file. It has some limitations, most notably not being 
> able to
> capture native wayland windows and loss of transparency information.
> However, I think, it is still interesting.
>
> I created 2 screenshots from gnome-shell on wayland (RHEL-8), first 
> one [2] is screenshot
> as produced by this program, second one [3] shows what was actually 
> visible on VMs display.
>
> I have not yet tested it on Xwayland with other graphical environments 
> than gnome-shell.
> (But feel free to experiment with this program on your wayland desktop.)
>
>
> Some technical notes (from testing on gnome-shell):
> - Xwayland is xserver, which runs as wayland client in "rootless" mode 
> (see [4]),
>   in which root window has no pixel storage, but otherwise it still 
> exists.
>   (not to be confused with running xserver as non-root user, which is 
> sometimes also referred as rootless mode)
> - XGetImage for both root window and "Composite Overlay Window" 
> (defined in composite extension [5]) fail with BadMatch
> - XQueryTree [6] can be used on root window to obtain list of it's 
> children (x11 windows) in stacking order (bottom to top)
> - Image of individual top-level windows (children of the root window) 
> can be obtained by XGetImage [7],
>   but window has to meet following conditions (otherwise it fails with 
> BadMatch):
>   - class is InputOutput (not InputOnly)
>   - map_state is IsViewable
>   - area specified does not lies fully inside of the root window

* area specified lies fully inside of the root window

> - pixel data obtained by XGetImage do not contain alpha information 
> (btw. xserver (Xwayland) reports 24bit depth),
>   so transparent pixels became opaque (see window shadows by gnome 
> shell and shaped window (xeyes) on linked screenshots)
> - nor pixel data nor any other information about native wayland 
> windows can be obtained through x11 (Xwayland),
>   so they are completely missing on the screenshot (also areas of x11 
> windows covered by them become visible)
>
>
> This approach is far from perfect, but perhaps could be used as last 
> resort
> to support some sort of screenshot on Xwayland in graphical 
> environment agnostic way (needs to be tested).
> (Probably still better than array of black pixels after XGetImage on 
> root window fails...)
>
>
> [1] 
> https://github.com/zzambers/wayland-experiments/tree/main/x11-screenshot
> [2] 
> https://github.com/zzambers/wayland-experiments/blob/fff27096e89bcb76ee43329ce987ccbb66dc4099/Screenshot-gnome-x11.png
> [3] 
> https://github.com/zzambers/wayland-experiments/blob/fff27096e89bcb76ee43329ce987ccbb66dc4099/Screenshot-gnome-vm.png
> [4] 
> https://gitlab.freedesktop.org/xorg/xserver/-/blob/ca1dfdc9aa4b548de624d3a9af5147a998ba3d79/miext/rootless/README.txt
> [5] 
> https://www.x.org/releases/X11R7.5/doc/compositeproto/compositeproto.txt
> [6] 
> https://www.x.org/releases/current/doc/libX11/libX11/libX11.html#XQueryTree
> [7] 
> https://www.x.org/releases/current/doc/libX11/libX11/libX11.html#XGetImage
>

-- 
Zdeněk Žamberský
OpenJDK QE
Red Hat