Ok, found this for X windows:

FVWM Forums • View topic - Thumbnailing a Window on another Viewport

As to why you can't access a window (that is, take a picture of it when it is not on the same desk/page as the current viewport) is because windows that are not on the current page are unmapped. This is true when one changes pages/desks -- the windows are unmapped, and the windows on the current viewport are remapped.

So, I guess that means what I want is impossible? Although there are two workarounds:

• use xrandr to increase the virtual size of (each of the four) viewports

This is what works for me - first query to see which output you want, then execute:

$ xrandr -q
Screen 0: minimum 320 x 200, current 1024 x 600, maximum 4096 x 4096
LVDS1 connected 1024x600+0+0 (normal left inverted right x axis y axis) 0mm x 0mm
   1024x600       60.0*+
   800x600        60.3     56.2  
   640x480        59.9  
VGA1 disconnected (normal left inverted right x axis y axis)

$ xrandr --output LVDS1 --panning 1600x768

... see also X/Config/Resolution - Ubuntu Wiki

• Use xvfb for virtual render

Got this via x11 - Remote offscreen rendering - Server Fault, also see Xvfb - Wikipedia, the free encyclopedia for examples. This is what worked for me - each command goes in own terminal (as some of them are meant to be services)

# start xvfb as display 1 - note the "screen 0" there refers to "its own" screen 0; should set it to 24-bit color

Xvfb :1 -screen 0 1600x1200x24 -pixdepths 3 -fp /usr/share/fonts/X11/Type1

# start metacity on display 1 (would start gdm, but [Bug #598848: gdm no longer works with xvfb](https://bugs.launchpad.net/bugs/598848)
# needed to we have title bars, and possibility to move windows around 

DISPLAY=:1 metacity

# start application (here web-browser arora) again on display 1

DISPLAY=:1 arora

# start vnc server, which will bridge display 1 to port 5900
# note - this command exists, however starts a server in background 

x11vnc -display :1 -bg -nopw -listen localhost -xkb

# start vnc viewer on port 5900 - so we can see display 1:
# note also - when vncviewer exits, it also will kill the x11vnc server

vncviewer -FullColor=1 localhost:5900

The problem here is that not all parts of the OS are initialized - so there are no window decoration, and only basic X fonts; although, apps may find their own fonts, as seen below:

xvfb looks almost ideal to me - since I could basically set up an "invisible" environment, as big as I want, and take screenshots directly from it (using xwd). The problem is that I cannot have a window on "my" DISPLAY=:0, then switch it to the 'virtual' DISPLAY=:1 for screenshotting, then bring it back to DISPLAY=:0. Then one must set up an application specifically for xvfb; and if so happens you need more size - then the whole chain above needs to be restarted.

On the other hand, I find working with 4 workspaces and large viewports (shwere the mouse moves you around within a viewport) a bit confusing - then again, can change the size there on the fly and nothing needs to be restarted; and plus I can use any of the usual screenshot tools (but I'd still have to switch viewports).

Needless to say, I'd still love to hear about a possible command line solution that could do this kind of screenshotting in one go :)

You can use VLC or scrot for this task.

Using VLC

This method allows you to make a screenshot using VLC, which is a very common program. To install VLC, use sudo apt-get install vlc

1. Start up VLC and open your video
2. Make the video fullscreen ( double click the video or press F11 )
3. Go to the time or frame where you would like your screenshot and press pause (this isn't really necessary since VLC can take snapshots on the fly, but this will help you get an exact frame)
4. Right click anywhere on the video, and go to Video -> Take Snapshot
5. Your video will be saved in the default snapshot directory ~/Pictures/ as xxxx-xx-xx-xxhxxmxxs.png (the directory to save screenshots can be changed by going to Tools -> Preferences -> Video -> Video Snapshots and changing the directory setting.

Using Scrot

This method isn't as elegant as the one above, but it will work with most video players. Scrot isn't installed by default, so you may have to use sudo apt-get install scrot.

1. Pause your video player.
2. Open terminal
3. Issue the command sleep 10 && scrot /path/to/new/image (obviously change /path/to/new/image)
4. Quickly change to your video in fullscreen
5. Wait

Using Scrot + Keyboard Shortcut

Now, this method can be quite annoying to use, so let's bind it to a shortcut.

1. Open System Settings (This can be found in the dash.
2. In the Hardware section, go to Keyboard -> Shortcuts -> Shortcuts
3. Now click the plus "+" button, and give your new command a name (e.g. "Screenshot"), and for the command type in scrot -e 'mv $f /your/pictures' (As always, change the directory to your liking)
4. Now click on where it says "Disabled" next to your command, and the text should change to "New accelerator...", enter your new keystroke in.
5. (Optional) If you want to remove the existing keystrokes for PrintScreen, you can click on the left side where it says "Screenshots" and remove each key-binding manually.

You can look scrot's documentation for more powerful features. man scrot