Real-time preview of Xvfb display using ffmpeg (w/ x11grab) and mpv. Note: ffplay works but offers too few options compared to mpv, notably the choice of fullscreen monitor via cli.

realtime-xvfb.sh

xvfb_display=':99.0'
xvfb_width=1920
xvfb_height=1080
xvfb_fps=30
xvfb_format='yuv420p'
ffmpeg \
  -hide_banner \
  -f x11grab -i "$xvfb_display" -video_size "${xvfb_width}x${xvfb_height}" -framerate "$xvfb_fps" \
  -f rawvideo -pix_fmt "$xvfb_format" - |
mpv \
  --no-config --no-terminal --no-osc --no-stop-screensaver \
  --demuxer=rawvideo --demuxer-readahead-secs=0 --cache-secs=0 --untimed \
  --demuxer-rawvideo-w="$xvfb_width" --demuxer-rawvideo-h="$xvfb_height" --demuxer-rawvideo-fps="$xvfb_fps" \
  --demuxer-rawvideo-mp-format="$xvfb_format" \
  --no-audio \
  -- -