[winswitch] issue with forwarding x to a lower res monitor from a virtual Xvfb

Zenny garbytrash at gmail.com
Mon Nov 16 12:30:27 GMT 2015

On 11/16/15, Antoine Martin <antoine at nagafix.co.uk> wrote:
> On 15/11/15 21:15, Zenny wrote:
>> Please find some more updates:
>> §1 If I use Xdummy with xpra (as described in the wiki) and try to
>> capture with ffmpeg the virtual console at 4k while displaying the
>> remote in 1080p, it categorically reports as of below and fails to
>> capture:
>> [x11grab @ 0x2b2dc00] Capture area 3840x2160 at position 0.0 outside
>> the screen size 1920x1080
>> :10: Invalid argument
> That's expected: Xdummy supports RandR and we resize the server's
> virtual screen to match the size of the client's screen.

Actually I want the server's virtual screen to remain at the same 4K
resolution while downsizing the one of the client's screen to say 1K
(in order to save bandwidth and for other reasons if any).

> That's one of the main reasons why you should avoid using Xvfb.
> You can verify this by running xdpyinfo on that virtual display (or
> "xrandr" or whatever).
> You can disable this behaviour using:
> XPRA_RANDR=0 xpra start ...
> As long as you're running a very recent beta build.
> You will also need "sync-xvfb" to make screen capture tools work:
> http://xpra.org/trac/ticket/988

The track specifies:

xpra start --sync-vfb=50 and there is no mention in the man page about
--sync-xvfb. Is there a some info about the the --sync-xvfb flag and
its parameters about usage?!

>> §2 If I use Xvfb directly with a single application (say xterm), and
>> run 'xpra start --use-display :10', it allows to screencapture, but
>> the capture of the window is dead black. (see attachment)
> Use "sync-xvfb".
>> §3. As in §2 above, if I start a DE like (LXDE or i3), I get the
>> xrandr errors and the remote screen freezes.
> Use Xdummy, Xvfb does not support RandR.

Yep, following your suggestion, I tried to use the xpra_Xdummy, but
that is nowhere found in my machine which is running v0.16.0-1 beta.

Thanks again for your very useful inputs.


> Cheers
> Antoine
>> Thanks!
>> On 11/15/15, Zenny <garbytrash at gmail.com> wrote:
>>> On 11/15/15, Antoine Martin <antoine at nagafix.co.uk> wrote:
>>>> I have tested here with Xdummy and that worked fine.
>>> Can you share your Xdummy command if different from the one in the wiki?
>>>>> I checked Xdummy related to Ubuntu instruction at
>>>>> https://www.xpra.org/trac/wiki/Xdummy, but to no avail.
>>>> Are you using Xvfb or Xdummy? What distribution and version?
>>>>> Any hints would be highly appreciated. Thanks!
>>>> You may also be interested in the following 0.16 features:
>>>> * ability to NOT change the virtual display to match the client's
>>>> resolution using:
>>>> XPRA_RANDR=0 xpra start ...
>>>> See:
>>>> http://xpra.org/trac/changeset/11229
>>>> Shadow mode is A LOT less efficient than the regular "start" mode.
>>>> * desktop scaling, which allows you to upscale or downscale the server
>>>> resolution when displayed on the client:
>>>> http://xpra.org/trac/ticket/976
>>>> So you could render at 4k and display at 2k for example (though the
>>>> main
>>>> purpose of this ticket is to do the opposite to save bandwidth).
>>>> * "sync-xvfb" fix for screen capture tools:
>>>> http://xpra.org/trac/ticket/988
>>>> Cheers
>>>> Antoine
>>>>> Cheers,
>>>>> /z
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