GLOSSARY
Mesh Normals
A normal is a vector perpendicular to a face that points outward from the mesh. Normals tell the renderer how light reflects, and they tell the slicer which side of each face is the inside of the part.
Definition
Each triangle in a mesh has a normal vector — a unit-length arrow pointing away from the surface on one side. Vertex normals are interpolated across faces for smooth shading. STL stores per- face normals explicitly; OBJ and glTF can store both.
The convention is that normals point outward from the volume the mesh encloses. If a face is flipped — its normal points into the mesh instead of out of it — the surface appears black or inside-out in viewers, and slicers may treat the volume on the wrong side as solid.
Why it matters
Flipped normals are one of the most common mesh errors — easy to introduce in Blender by accidentally extruding inward, and easy to ship with AI-generated meshes that do not enforce consistent orientation. The fix is one click: Blender's "Recalculate Normals" (Shift+N) or MeshLab's "Re-orient all faces coherently" filter.
Common confusion
STL stores both face normals and vertex order. The two should agree — the right-hand rule on the vertex order should produce the stated normal. Some exporters get this wrong, and slicers rely on different parts of the data, leading to inconsistent behavior between tools.
For visual rendering, "normal maps" are a different concept: 2D textures that encode small surface variation without adding geometry. Useful for game assets, irrelevant for printing.