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Using Xvfb I can launch an application and take a screenshot.

What I'm wondering about is how to capture from an existing (dormant) X session.

Say from tty or over ssh.

From what I understand windows are unmapped and does not exist to preserve resources. Are there some tool that can map existing windows, workspaces etc in a virtual windowmanager and take a snap?

Or can one perhaps use libx11 and create a snap by using XShmCreateImage or similar?

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Is xwd -root | convert - /tmp/image.jpg what you are looking for? –  jippie Dec 8 '12 at 21:31
@jippie: Perhaps. Problem is that as windows are unmapped there is nothing for xwd to dump - resulting in black image. (Try that command from i.e. tty1 xwd -display :0 -root | convert xwd:- /tmp/0.png) –  Sukminder Dec 8 '12 at 21:38
unsure what you mean by 'unmapped' –  jippie Dec 8 '12 at 21:38
@jippie: They do not reside in memory. As I understand it: When you are on your workspace - it is active - the pixels are mapped in memory for the GPU to process and render on screen. When workspace is switched or exited (I.e by Ctrl-Alt-F1) these maps are unmapped (to free up resources) - thus there is nothing in the "display buffer" - and gives a black image. –  Sukminder Dec 8 '12 at 21:45
I see what you mean, thnx for the explanation. Don't know the answer in this case though. –  jippie Dec 8 '12 at 21:48
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1 Answer

up vote 5 down vote accepted

We can "turn on" the X tty manually from command line, without ctrl-alt-F(x).

If X is on tty 7, and you login a text terminal

chvt 7 ; DISPLAY=0.0 xwd -root > screendump.xwd ; chvt 1

However, xhost + localhost must be issue within the xsession, else xwd cannot connect to x server.

This allow you to do it from ssh session.

PS: In theory, with MTD, it should be possible to do screen capture from video card memory. Thus without first "waking" the X session.

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Thank you for a good post. I'm looking into it. One of my major issues is that I am an old windows'er and having some problems grasping how everything is knitted together. X server, X window manager, access and composing. But getting there. –  Sukminder Dec 14 '12 at 19:24
@Sukminder Do you have difficult setting this up or need more instruction regarding this? –  John Siu Dec 17 '12 at 15:44
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