Re: live copy of an X screen

Arnaud Loonstra <arnaud@xxxxxxx> · Sat, 28 Jan 2012 12:40:59 +0100

On 01/27/2012 11:45 PM, Matthew Monaco wrote:

On 01/27/2012 11:13 AM, Arnaud Loonstra wrote:

Hi all,

I've been searching for a while for this but don't seem to find a
solution. My
quest is: Is it possible to get a copy of an X screen so I can display it
somewhere else?

Suppose I have two X screens :0.0 and :0.1. Would it be possible to
capture :0.1
and display it in a little window on :0.0? This would be extremely
useful for
running things fullscreen on a second display (:0.1) while controlling
it from
the first (:0.0) while being able to see the contents of the second on
the first
display.

I could write some capture code and get a framebuffer this way but I
thought I
would first explore if this isn't somehow built in already or done
before?

Any thoughts/suggestions?

Rg,

Arnaud Loonstra

I use x11vnc / tightvnc

I know it's an option however the overhead is way too high. The perfect 

solution would be to do it in the gpu. Everything is there so why 

process it in the cpu. In openGL you can do similar things with render 

to texture code.

I'm not familiar with any X11 api but there seem to be calls to just 

grab a display. I've been browsing through the ffmpeg code

http://git.videolan.org/?p=ffmpeg.git;a=blob;f=libavdevice/x11grab.c;h=722f00ff0df14848e9c4202fd63360f60a7ee1ee;hb=HEAD
Which is giving some pointers... the Display struct, Xshm?

I've tried how much overhead such code would take by issuing
ffmpeg -f x11grab -r 10 -s xga -i :0.0 -an -f rawvideo /dev/null

which takes some 10% of CPU. This is still too much... In this case 

ffmpeg is still passing the data through. No encoding..

Input #0, x11grab, from ':0.0':
  Duration: N/A, start: 1327750724.645929, bitrate: 251658 kb/s

    Stream #0.0: Video: rawvideo, rgb32, 1024x768, 251658 kb/s, 10 tbr, 

1000k tbn, 10 tbc

File '/dev/null' already exists. Overwrite ? [y/N] y
Output #0, rawvideo, to '/dev/null':

    Stream #0.0: Video: rawvideo, rgb32, 1024x768, q=2-31, 200 kb/s, 

90k tbn, 10 tbc

Stream mapping:
  Stream #0.0 -> #0.0

Rg,

Arnaud Loonstra

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