Kinda related, here's a few ffmpeg commands i've probably spent many hours getting to work just right recently (and they might not even the right way to do these things), perhaps someone will find it helpful. All of these are on a macbook.

- play facetime camera (or any avfoundation camera, like a capture card) to mpv

  ffmpeg -f avfoundation -video_size 1920x1080  -framerate 30 -drop_late_frames true -i "FaceTime HD Camera" -c:v rawvideo -f matroska -r 30 - | mpv -profile=low-latency -untimed -

- play raw video stream from the usb camlink capture card

  ffmpeg -f avfoundation -video_size 3840x2160 -framerate 23.98 -pixel_format uyvy422 -drop_late_frames true  -i "Cam Link 4K" -c:v rawvideo -f matroska - | mpv -profile=low-latency -untimed -

- Capture video, and do live HVEC hardware encoding on apple hardware and save result. Control quality with q variable. q=85 is around 250Mb/s:

  ffmpeg -f avfoundation -video_size 3840x2160 -framerate 23.98 -i "Cam Link 4K" -c:v hevc_videotoolbox -q:v 85 -r 23.98 -tag:v hvc1 <filename>_x265.mp4

- Use named pipe with mkfifo to play a stream. Sometimes this produces less input lag than other methods.

  mkfifo /tmp/fifo1
  cat /tmp/fifo1 | mpv -profile=low-latency -untimed -
  ffmpeg -f avfoundation -video_size 1920x1080  -framerate 30 -drop_late_frames true -i "FaceTime HD Camera" -c:v rawvideo -f matroska -r 30 - > /tmp/fifo1