Example of a domain decomposition with two shell data domains (pink and blue overlapping areas) covering a spherical computational domain (solid black circle). A photon packet emitted from a central source would first propagate through data domain 1 (pink), then be transferred to the data domain 2 (blue), and eventually escape the computational domain. The large overlap between domains 1 and 2 (purple) ensures that the photon is well within domain 2 before being transferred, which reduces possible communication overheads due to photons oscillating between domains.