Fig. 2

During the expansion of clusters (clouds of spatially connected voxels) in 3D, a cluster can grow via those voxels that are sharing a common edge with the one of existing cluster’s voxels. This property is regulated by parameters of \( \epsilon =\sqrt{2},\ Minpts=2 \) . In this 3D figure, each cube represents a voxel. The centered (red-outlined) voxel (#1) and only five (#2, #3, #4, #5, #6) of its twelve common-edge neighbors are depicted for the sake of simplicity. Note that point-based neighborhood (sharing only one corner, such as #1 and #7) between two voxels does not satisfy the condition of cluster expansion