Degenerate geometry

Degenerate geometry is geometry that has collapsed: a face with zero area, or an edge with zero length. Blender's 3D Print Toolbox uses the word degenerate for exactly these faces and edges, and suggests merging by distance to fix them (Blender 3.6 Manual: 3D Print Toolbox). The glTF specification says mesh geometry "SHOULD NOT contain degenerate lines or triangles, i.e., lines or triangles that use the same vertex more than once" (Khronos Group: glTF 2.0 Specification). A degenerate triangle is one whose three corners lie on a single point or a single straight line.

Degenerate geometry is usually invisible in the viewport, and only its side effects show.

#How it happens

Typical causes, all of which leave the faces in place but flattened:

• Extrude, then cancel. In Blender, cancelling the move after an extrude keeps the new faces, stacked on the old ones (see extrude, inset and loop cut).
• Snapping vertices together without welding. Two vertices at the same position give zero-length edges between them; Merge by Distance exists to weld exactly these (Blender 3.6 Manual: Clean Up).
• Booleans and conversions. Cuts and format conversions can leave slivers; see booleans.

#Problems caused by degenerate geometry

A face's normal is worked out from its edges. When the face has no area, there is no sensible direction for it to face, so every tool that needs one has to guess. Problems appear wherever normals matter: smoothed shading (see smoothing, hard edges and smoothing groups), baking and tangent space and MikkTSpace, and anything that needs a clean solid, such as booleans and 3D-print checks (Blender 3.6 Manual: 3D Print Toolbox). See manifold and non-manifold geometry and fixing baking artefacts.

Unreal's FBX importer has a Remove Degenerates option, and Epic says "this option should be enabled" in general (Unreal Engine Documentation: FBX Import Options Reference).

#Finding and fixing it

Blender's Clean Up menu also has Delete Loose, which "removes disconnected vertices and edges (optionally faces)", and Split Non-Planar Faces, which "avoids ambiguous areas of geometry by splitting non-flat faces" (Blender 3.6 Manual: Clean Up). They are worth running in the same pass.

#Common mistakes

• Merge distance too large. A big threshold also merges vertices you meant to keep, such as tight support loops or small bevels. Start very small and increase only if needed.
• Cleaning after baking. Clean first, then bake; the bake depends on the low poly's normals.
• Trusting the viewport. Degenerate faces are invisible until something breaks. Run the check before every export.

Related: manifold and non-manifold geometry, triangle, face, normals, extrude, inset and loop cut, export checklist.

#Further reading and watching