Distributed grid with subgrid copies. The subgrids in green and orange represent the same portion of the grid, but are not linked during the photon-propagation step. They each store information about their neighbours as if they were a regular subgrid. However, their neighbours only store information about one of the copies, so that the neighbours themselves are also unaware of the subgrid copy. In addition to creating the subgrid copy, this rerouting of the subgrid interconnections is the only step that is required to use the subgrid copies in the task-based algorithm and imposes very little overhead.