Question about multiple X11 and screencaptures

[Barry Scott]

On Wednesday 29 September 2010 12:27:08 Marc Cambier wrote:
> Hello,
> 
> I have a command 'cmd', which takes a screencapture.
> 
> I have two X running :
> - Display :0 on VT2
> - Display :1 on VT3
> 
> I go to VT2 : CTRL+ALT+F2
> I launch cmd : I have screenshot of the display :0, it's OK
> 
> I go to VT3 : CTRL+ALT+F3
> I launch cmd : I have screenshot of the display :1, it's OK
> 
> But, I have an X program running on display :1, which takes screencaptures
> while I'm using display :0.
> 
> And that doesn't work, for example :
> I go to VT3 : CTRL+ALT+F3
> I launch "sleep 20 && cmd"
> During the sleep, I go to VT2 : CTRL+ALT+F2
> I wait 20 seconds... Screenshot is done, but the result image is totally
> black.
> 
> So, I suppose something makes the display :1 "blank" when I'm on display :0
> ;
> I searched around the web, but didn't find any explication...
> 
> Pleaz,
> Could someone explain me this behaviour ?

You have one set of hardware to draw on.
Each time you switch VT the hardware is given to the VT.
You can only get pixels from the hardware for the VT that is active.
The inactive VT's cannot return pixels.

> Is there a way to get rid of this problem ?

> 
> I can provide you more informations if you ask...
> 
> Thanks.
> M.
> 

Barry