Non-Manifold Geometry
Mesh that cannot exist as a real solid
Also known as: Non manifold · Non-manifold edges · Loose geometry · Interior faces
Definition
Non-manifold geometry is mesh that could not exist as a real solid object, such as edges shared by three or more faces, loose vertices and edges, faces inside the model or zero-area faces. It causes boolean operations, remeshing, subdivision, 3D printing and baking to fail or produce artifacts. You usually fix it by merging by distance, deleting loose parts and removing interior faces. In Blender, Select All by Trait > Non Manifold highlights the problem areas.
In production
Check for it before a boolean, a remesh or a bake, and after importing a mesh from another tool or from photogrammetry. Clean up with Merge by Distance (Merge Vertices in Maya), delete interior faces and fill holes. Game meshes may have intentionally open edges, such as a wall plane, so non-manifold is not always an error, but a high-poly used for baking should be clean.
