Bearing-only control of multiple robots in 3D
These video show real experiment of a control method that allows a group of mobile robots (e.g., quadrotors) to achieve the desired formation only resorting to bearing (angles) measurements, like the ones that are retrievable from an on-board camera.
More info on the metho: Franchi A, Masone C, Grabe V, Ryll M, Bülthoff HH, Robuffo Giordano P. Modeling and Control of UAV Bearing-Formations with Bilateral High-Level Steering. The International Journal of Robotics Research, Special Issue on 3D Exploration, Mapping, and Surveillance. 2012 ;31(12):1504-1525.