Or you can use x11vnc to enable remote access by VNC client:

x11vnc -display :0 -auth /var/run/xauth/A:0-*

-- 
On Tuesday 05 of February 2019 02:26:00 Alexandre Couture wrote:
> Hi,
>
> One way to do a screenshot of TDM is to run it in VirtualBox, and then
> take a screenshot with the host OS (can be linux too).
>
> Have a great day!
> -Alexandre
> ________________________________
> De : Sl�vek Banko <slavek.banko@...>
> Envoy� : 4 f�vrier 2019 19:06
> � : trinity-users@...
> Objet : Re: [trinity-users] want TDM screenshot
>
> On Tuesday 05 of February 2019 00:38:11 Felix Miata wrote:
> > Sl�vek Banko composed on 2019-02-04 23:02 (UTC+0100):
> > > Felix Miata wrote:
> > >> https://www.maketecheasier.com/take-screenshot-of-login-screen-lin
> > >>ux / is the closest I have found to instructions to use ImageMagick
> > >> to get one, but tdm seems to be different from lightdm and gdm
> > >> such that I can't figure out what to change. I tried logging in
> > >> and grepping XAUTHORITY from set, but it apparently cannot be
> > >> found there.
> > >>
> > >> I want a fullscreen screenshot that has the integrated default
> > >> theme's menu expanded to show the selection of session types. Can
> > >> anyone here help?
> > >
> > > You can search in: /var/run/xauth/
> >
> > I don't see any connection. e.g. in Stretch I see (only):
> >
> >        /var/run/xauth/A:0-TZtcAa
> >
> > 44 binary bytes owned by root and containing string
> > "MIT-MAGIC-COOKIE-1". On openSUSE it's the same except for the last 6
> > characters of the filename. How can that be translated to
> >
> > /var/lib/gdm/$DISPLAY.Xauth
> > or
> > /var/run/lightdm/root/$DISPLAY
> >
> > ???
> >
> > Would it be simply
> >
> >        /var/run/xauth/$DISPLAY
> >
> > or something else?
> >
> > I tried putting
> >
> >        #! /bin/bash
> >        chvt 7
> >        DISPLAY=:0 XAUTHORITY=/var/run/xauth/$DISPLAY xwd -root
> >
> > in the script, but the resulting output image is always 0 bytes, and
> > error messages:
> >
> >        No protocol specified
> >        xwd: unable to open display ':0'
> >
> > I tried on Tubuntu 18.04/14.1 with same result.
>
> First, for XAUTHORITY should be:
> XAUTHORITY=/var/run/xauth/A${DISPLAY}-*
>
> Second, I do not see in the man page that xwd would accept the
> environment variable XAUTHORITY or option like -auth or -xauth.
>
> Cheers
> --
> Sl�vek