4. Hard edges and UV seams

Split vertices do not break the shading. Split normals do.

Segments

Normals on the seam Matched (average faces on both sides) Separate (each side averages its own faces)

Light direction (left/right) Light height Toon shading Toon threshold Show vertex normals

Vertices: · Drag the view to turn it.

This sphere has split vertices along the vertical line in front (the same as a UV seam).
Matched: the split vertices share the same normal, so the shading does not break. Being split is not a problem in itself.
Separate: each split vertex averages only the faces on its own side. The two sides point differently, so a shading line appears along the seam. Turn the light with toon shading to see it clearly.
With “Show vertex normals” you can see whether each seam point has two normals or one.