Screenshot of a virtual terminal when it is not focused (in the background)

Question

This is on Ubuntu. I have two xsessions, one on tty7 (display :0) and one on tty8 (display :1). For the record, :0 is Unity and :1 is Fluxbox. I would like to be able to take a screenshot on display :1 while working on display :0.

I'm using the Java Robot class. I've written a small screen scraping bot. The mouse movements work when I'm switched to the other virtual terminal, but the screengrabs are coming out all black (the bot has no image to act on).

I'm spawning the second xsession with:

startx /usr/bin/fluxbox -- :1

Is there a way to make the screenshots work? Is there a workaround for keeping the display active (remote desktop to it?)? Would someone be kind enough to explain why these go black when the virtual terminal is not focused?

I'm looking for the least intensive way to have a second display, mouse, and keyboard running. The path of least resistance would be a way to keep the display active when not focused.

Thanks! Sorry if any of my terminology is off, new to using a second virtual terminal.

Answer 1

You get a black image simply because the image you want to capture does not exist anywhere.

When a virtual terminal is active, each program that is on your display will generate its own image, and the image gets put into the video memory. When the virtual terminal is not active, the programs do not generate their image (since it would be superfluous), nothing stores those images, so you cannot capture them.