  In this paper, we present a novel 3D formation control scheme for directed
graphs in a leader-follower configuration, achieving (almost) global
convergence to the desired shape. Specifically, we introduce three controlled
variables representing bispherical coordinates that uniquely describe the
formation in 3D. Acyclic triangulated directed graphs (a class of minimally
acyclic persistent graphs) are used to model the inter-agent sensing topology,
while the agents' dynamics are governed by single-integrator model. Our
analysis demonstrates that the proposed decentralized formation controller
ensures (almost) global asymptotic stability while avoiding potential shape
ambiguities in the final formation. Furthermore, the control laws are
implementable in arbitrarily oriented local coordinate frames of follower
agents using only low-cost onboard vision sensors, making it suitable for
practical applications. Finally, we validate our formation control approach by
a simulation study.
