The Join Meshes   icon on the Geometry panel activates a utility to join the unmatched edges in a set of surfaces that are topologically open. When surfaces remain open, usually after importing a low-resolution surface mesh, this utility is used to repair the open edges. This leaky mesh problem can be identified by red highlights on surface edges, as seen in Figure 3.11: (a) Red edges indicate a leak in the surfaces that define a watertight mesh. (b) Relaxing duplicate edge tolerance in a mesh join often repairs this issue, and can often be remedied by relaxing the duplicate edge tolerance. Click the Join Meshes   icon to change this setting.

Figure 3.11: (a) Red edges indicate a leak in the surfaces that define a watertight mesh. (b) Relaxing duplicate edge tolerance in a mesh join often repairs this issue