Smoothing Groups
Per-face shading split
Also known as: Smoothing normals · Hard edges · Smoothing
Definition
Smoothing groups (called "hard edges" in Maya/Blender) control which faces of a mesh share averaged vertex normals — making them shade smoothly — versus which faces produce a sharp lighting break. They are the single most important per-vertex attribute beyond position and UV: identical geometry with different smoothing settings will look like two completely different surfaces under lighting. Always set hard edges along UV seams to avoid baking errors.
In production
Standard rule: every UV seam should also be a hard edge, and every hard edge should be a UV seam. This avoids the gradient artefacts caused by synced vs non-synced tangent-space normal bakes. Marmoset Toolbag exposes this in its baker as "Use Hard Edges".
