Using a laptop as a second monitor (Linux)

So first thing you need is an HDMI capture card. I got myself a cheap one for £5 on Amazon.

One thing you'll notice when simply using mpv /dev/video4 is that it defaults to yuyv422 at 5fps. The same happens in OBS and VLC. Using the command from the Arch wiki mpv --demuxer-lavf-format=video4linux2 --demuxer-lavf-o-set=input_format=mjpeg av://v4l2:/dev/video4 gets you much better fps at the cost of high latency (Up to 1000ms).

mpv --demuxer-lavf-format=video4linux2 --demuxer-lavf-o-set=input_format=mjpeg --profile=low-latency --untimed av://v4l2:/dev/video4

This gets me much better latency (50ms) with lower resolution but still very usable for watching videos and reading text. To improve resolution, I lowered the FPS to 30 in Hyprland

monitor=HDMI-A-1,1920x1080@30.00Hz,auto-right,1.5