They fixed the observers head to avoid motion parallax and used three lights in a triangular pattern (symmetric, low intensity in a dark setting, along a eye-level plane). A fourth light is introduced and the observer as asked to place the light on the left, equidistant from the two endpoints and then a fifth light on the other side. This is locating those midpoints.
They fixed the observers head to avoid motion parallax and used three lights in a triangular pattern (symmetric, low intensity in a dark setting, along a eye-level plane). A fourth light is introduced and the observer as asked to place the light on the left, equidistant from the two endpoints and then a fifth light on the other side. This is locating those midpoints.