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I'd like to have a way of taking a portion of some window (e.g. the video on a youtube page, or the progress bar of some program) and "streaming" just that part to a separate floating window so that I can see just that bit on another desktop.

I know that streaming software like OBS can do something like this, and I know that ffmpeg/ffplay can do this in theory (I could not get any examples to work). I also feel like it should be possible for the compositor to do this (like live alt-tab previews) but I couldn't find a suitable compiz plugin.

How can I get a second, separate view of a desktop program (ideally with scaling)?

(I'm on a standard install of Ubuntu Bionic.)

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    Opera can do this for video streams (e.g. Youtube) – Nordlys Jeger Aug 19 '18 at 23:11
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Have a look at the xmag or xmagnify X applications; they read out part of the framebuffer, magnify it, and display it in a separate window.

This will not work for videos displayed in the video overlay (which is rarely used today, as overlays now mostly use the GPU to render the surface).

Also note that these applications are pure X and predate compositors, I have no idea how they'd interact with a compositor.

Also, viewing the window "on another desktop" may not work, as windows not on your current desktop are unmapped and won't update.

It would probably be possible to modify the source to directly read from the window backing store of the application you are interested in.

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