The Ball Pointer is a computer vision system, that allows determining a physical ball's position, up to scale.
The basic idea is simple, the further away from the camera the ball is, the smaller is will be, giving an indication of the ball's distance from the camera. This can be formalized using linear algebra, and then it becomes a problem of detecting the ball in an image, for which there are already many solutions.
The Ball Pointer is used by Solid Editor, as a means of drawing voxels in 3D, but could be used for many other purposes, e.g. as a cheap replacement for PS Move/Kinect/Wiimote in games. If you want to use it in one of your applications today, you can get early-access to the code by making a one-time GitHub Sponsors donation. It will be open sourced sooner or later though, probably together with the rest of Solid Engine.