I have built something similiar to the follwing to track the head:
https://chris.gunawardena.id.au/software-development/webgl/optical-position-tracking-with-opencv-and-aruco-markers/strong text
Lets say i want to know the Position of the nose and track it. How do i get the 2D coordinates on the Image for that?
For Example the nose is 100mm below the marker and 80 mm towards the camera.