A shading rule keyed on a grid-sampled slope renders narrow features as a row of teeth

Fact

Height fields are commonly shaded by rules that are steep functions of slope — snow does not settle on a steep face, exposed rock reads darker. Slope is sampled per vertex from neighbouring heights, so it carries the grid's resolution.

Where a narrow feature crosses the grid at an angle — a cut bank, a ridge, a crack — its phase against the grid changes along its length. A linear use of that slope hides it. A steep smoothstep amplifies it into a regular row of teeth marching along the feature.

The tell is that the geometry and the normals are perfectly smooth and the colour is not. Rendering the same mesh with a normal-visualising material shows nothing wrong; the artifact lives only where a threshold was applied.

Why it matters

Teeth look like a mesh problem, so the search starts at resolution, triangulation and computeVertexNormals — all of which are innocent, and none of which fix it. Raising the mesh resolution makes the teeth smaller and more numerous, which reads as progress and is not.

How to apply

• Split the two uses of slope. Keep the raw per-vertex slope for anything linear; low-pass a copy — one 3x3 average over the grid neighbours is enough — for anything that goes through a threshold.
• Widen the threshold band while you are there. A rule that switches over a narrow range of slope will find whatever high-frequency content exists.
• To confirm the diagnosis in one step, swap the material for a normal-space one. Smooth normals with teeth in the shaded render means the fault is in the colour, not the mesh.