Model geometry should not contain non-manifold vertices. A non-manifold is a 3D mesh that can not be unfolded into a 2D surface with all its normals pointing in the same direction.
Non-manifold geometry examples
- Non-manifold “T-type”
- Internal faces
- Surfaces connected to one vertex
- Loose geometry
- Opposite normals
How to solve
- Detect non-manifold vertices/edges in Blender.
- Select a geometry object.
- Enter Edit Mode by pressing Tab key.
- Use Vertex or Edge select mode.
- Make sure that all vertices/edges are deselected.
- Go to Select → Select All by Trait → Non Manifold
- Deselect Boundaries from the Select Non-Manifold menu. (Geometry with open edges is acceptable)
- When non-manifold vertices/edges are detected issue can be fixed by:
- Deleting/separating faces which which have non-manifold vertices/edges.
- Deleting loose geometry (vertices/edges).
- Flipping normals of the adjacent faces which has normals pointing in opposite directions.
- Merging non-manifold vertices.