Parallelization of the watershed segmentation shown on a 2D field. The top panel depicts the computational box with the density field. In the second panel, the two MPI domains and the virtual boundaries are shown, the peaks have obtained their IDs and the cells are labeled. In a loop over all test cells, the peak patch labels can propagate inside the MPI domains (third panel). After the loop, the virtual boundaries are updated and the procedure is repeated (fourth panel).