Each vertex in polygonal mesh belongs to none (isolated vertex) or several polygons. All the polygons that are adjacent to a given vertex and share a common edge form a fan. The topology is manifold if for each vertex there is exactly one fan.
Having a non-manifold topology in production basemesh is often not a good practice unless you know what you are doing. Many times non-manifold meshes are created by mistake and can later cause problems with editing, rendering etc.
Below are some examples of non-manifolds:
Non-manifold cases include:
- singular vertex:
more than two fans are adjacent to the given vertex
- non-manifold edge:
more than two polygons are adjacent to single edge
- unoriented polygons:
two polygons that share a common edge have opposite orientation of normals
- isolated vertex:
vertex that does not belong to any polygon
- invalid polygon:
polygon that includes a given vertex multiple times
Some nodes in Wrap like DeletePolygons can produce non-manifold topology.