Weighted (custom) normals

Weighted normals (also face-weighted normals, FWN, or custom normals) are vertex normals calculated so that large faces count more than small ones. On a bevelled mesh, the big flat faces keep their own direction and the thin bevels do all the bending, so flat faces shade flat. Blender's modifier "changes the custom normals of a mesh" and is often used to make some faces look flat during shading (Blender Manual: Weighted Normal Modifier).

#Shading problems on bevelled meshes

By default, a smooth-shaded vertex gets a normal blended from all faces around it (see normals). Put a small bevel between two big faces and each bevel vertex gets a normal halfway between the big face and the bevel. The renderer interpolates that tilted normal across the big face, which looks like a dark smudge or a dent near every edge.

The old fixes were support loops (more geometry) or hard edges plus a normal map bake. Weighted normals fix the shading by changing only the stored normals: same vertices, same triangles.

#How weighting works

The technique is not new. A 2011 Polycount thread noted that id Software had used the technique since Doom 3, and one member summarised it as: "Literally multiply vertex normal contribution by surface area of the face it belongs to" (Polycount forum: Discuss face weighted normals).

Blender's Weighted Normal modifier offers three modes (Blender Manual: Weighted Normal Modifier):

• Face Area: "A larger area means that the normal from that face will get a higher weight in final result." This is the mode for hard-surface work.
• Corner Angle: weights by the angle each face makes at the vertex; "This is the method Blender uses by default".
• Face Area and Angle: both multiplied together.

Other useful settings (Blender Manual: Weighted Normal Modifier):

• Weight: a contrast control. At 50 all faces count equally; higher values exaggerate the difference.
• Keep Sharp: preserves sharp edges.
• Face Influence: uses face strengths (weak, medium, strong) set by the Set Strength tool or the Bevel modifier, and lets only the strongest face decide. Patscheider recommends it for hard-surface modifier stacks (ArtStation blog: Quarry Bank Mill 3D #07 Modifiers and Wires /…).

#When you still need a normal map

Weighted normals fix shading of the shape you modeled. They do not add detail. You still need a normal map or other texture for:

• detail that is not in the mesh, such as panel lines, screws and dents (often from trim sheets or mesh decals);
• curved surfaces too low in density to shade smoothly;
• cases where a unique bake is cheaper than the extra triangles.

If you do bake onto a weighted-normal mesh, bake with the final custom normals. A participant in the 2011 thread said he would not add FWN to an object with an existing baked normal map, because it may add shading errors (Polycount forum: Discuss face weighted normals). On mid-poly assets, a common answer is that a separate high poly is unnecessary: "mid poly modeled with appropriate detail will serve as both the source and target mesh" (Polycount forum: Weighted Normals (Mid Poly) Workflow…).

#Export and engine import

Custom normals only work if the exporter writes them and the engine keeps them on import.

1. Export with normals. Export FBX with custom normals included, and triangulate consistently before export so the engine does not choose a different triangle split.
2. Import with normals. In Unreal Engine's FBX importer, Normal Import Method has three choices (Unreal Engine documentation: FBX Import Options Reference in Unreal Engine):
   • Compute Normals: "Smoothing group and normal info is discarded from the FBX info." Do not use this.
   • Import Normals: "The normals are imported from the FBX file, tangents are computed by the engine." The usual choice for weighted-normal meshes.
   • Import Normals and Tangents: nothing is computed by the engine.
3. Check under a moving light. Rotate a directional light and look for smudges on flat faces.

#In different tools

• Blender: Weighted Normal modifier, placed after Bevel (Blender Manual: Weighted Normal Modifier). In the 2.8x manuals it required Auto Smooth to be enabled first (Blender Manual: Weighted Normal Modifier); check the manual for your version. The Bevel modifier's Harden Normals and Set Strength options feed into it (Blender Manual: Bevel Modifier).
• 3ds Max: Weighted Normals modifier since 3ds Max 2021 (Autodesk Community: Implement a new Vertex Normal Modifier), improved in 2022 (Arkance Community: What's New in 3ds Max 2022? – New and…).
• Maya: Maya 2013 added a face-weighted normals option in the shape node's Mesh Controls (Polycount forum: Discuss face weighted normals). Artists still use plug-ins: Jessica Canales used GenTools (80 Level: How to Create a Detailed Large-Scale Scene…). Polycount users complain Maya's default weighting gives different results from Max and Blender, and lacks face-size priority control (Polycount forum: Maya and it's Weighted Vertex Normals).
• Modo: one artist in the Sunset Overdrive trim thread said he can "edit vertex normals with one click in Modo 10" (Polycount forum: The 'ultimate trim' technique from Sunset…).

#Further reading and watching