capturing video of the screen in X11

 Thanks to [0], one can capture a video of the top-left of the X11 screen (using this term in its general sense, not the X sense), with something like the following:

$ ffmpeg -video_size 1000x1300 -framerate 60 -f x11grab -i :0.0 -c:v libx264rgb -crf 0 -preset ultrafast -color_range 2 screencast.mkv

the technique being to move the window of interest to the top-left before starting recording.  The dimensions to record can be adjusted from the 1000x1300 given above.

[0] https://trac.ffmpeg.org/wiki/Capture/Desktop

How does one simply capture the whole screen without having to specify the dimensions?

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