Manifold and non-manifold geometry

A mesh is manifold when "every edge is shared by exactly two faces", with no holes and no faces cutting through each other, so it forms a closed surface with a clear inside and outside (Meshy: Manifold). Anything that breaks this rule is non-manifold geometry. People also call a manifold mesh watertight.

#What counts as non-manifold

Most problems fall into a few patterns.

• Edges shared by more than two faces. For example, an internal wall left inside a box, or a fin sticking out of an edge (Meshy: Manifold).
• Edges with only one face. These are open borders or holes (Meshy: Manifold). Blender's 3D Print Toolbox flags edges that connect to only one face or to more than two as non-manifold: if an edge "connects to only one, it means there's a hole in the mesh" (Blender 3.6 Manual: 3D Print Toolbox).
• Faces that touch only at a vertex. Two faces meeting at a single corner without sharing an edge do not form one continuous surface (Meshy: Manifold).
• Loose geometry. Stray edges or vertices with no faces (Meshy: Manifold).
• Duplicate and overlapping geometry. Unmerged vertices on top of each other, interior faces, and coincident surfaces left by booleans (Meshy: Manifold). Maya calls two faces that share all their edges lamina faces (Maya 2017 Help: Cleanup Options).
• Zero-area faces and zero-length edges. Faces or edges collapsed to nothing. Blender's toolbox calls these degenerate and suggests merging by distance to fix them (Blender 3.6 Manual: 3D Print Toolbox).
• Normals that disagree. Blender's toolbox also flags "Bad Contiguous" edges, where one face's normal points against its neighbours (Blender 3.6 Manual: 3D Print Toolbox). See normals.

#Problems caused by non-manifold geometry

Many tools need to know what is inside and what is outside. Non-manifold geometry makes that impossible to answer, and Meshy's glossary lists slicers for 3D printing, boolean tools and physics engines among the tools it confuses (Meshy: Manifold).

• Booleans. A boolean cuts one shape with another by working out which parts are inside which. Holes, internal faces and overlaps give it wrong or no answers (Meshy: Manifold).
• Subdivision. Maya's OpenSubdiv implementation does not support non-manifold meshes, so Maya falls back to its older Catmull-Clark method when a mesh contains non-manifold components (Maya 2022 Help: OpenSubdiv General limitations). OpenSubdiv itself has added non-manifold support for some schemes over time (CG Channel: Pixar releases OpenSubdiv 3.4), so behaviour depends on the tool and version.
• 3D printing. A printer needs a watertight solid it can slice into layers (Meshy: Manifold).
• Engine import. Unreal's importer can remove degenerate triangles, and Epic says "this option should be enabled" in general (Unreal Engine Documentation: FBX Import Options Reference). Removing them yourself means you know what changed.

#Common mistakes

• Booleans leaving coincident faces. Two surfaces in exactly the same place flicker in the engine and confuse later booleans (Meshy: Manifold). See booleans.
• Deleting faces but keeping their edges and vertices. You are left with wire edges and loose vertices (Meshy: Manifold).
• Ignoring a "Bad Contiguous" warning. It usually means a flipped face (Blender 3.6 Manual: 3D Print Toolbox). See normals.

#In different tools

• Blender. The 3D Print Toolbox runs a Solid check for non-manifold edges and bad normals, plus Intersections, Degenerate and Distorted checks, and offers a Make Manifold clean-up (Blender 3.6 Manual: 3D Print Toolbox). Its sharp-edge default also treats non-manifold edges as sharp (Blender 3.1 Manual: Mesh Structure). Turn on Face Orientation to spot flipped faces (Blender 3.5 Manual: Viewport Overlays).
• Maya. Mesh > Cleanup can remove lamina faces, non-manifold geometry (with an option to also fix normals), zero-length edges and zero-area faces (Maya 2017 Help: Cleanup Options).
• Unreal Engine. Remove Degenerates in the FBX import options (Unreal Engine Documentation: FBX Import Options Reference).

#Related terms

edge · face · normals · booleans · topology · remeshing · baking

#Further reading and watching