Capture image from jit.window

I'd like to know the best way to grab an image from a jit.window, using jit.gl.render, and send it to a jit.matrixset.
Actually, I'm using jit.desktop to localize the window and grab it from there, but it makes my video jitter a bit when I take the snapshot... I would like to have a way to get a snapshot of jit.window without influencing the video going through... Maybe with matrix output in gl.render, but I don't want to skip some frame.

Any help would be greatly appreciated!

Thanks.