= Lateral artifacts of a subdivision surface
<Lateral artifacts of a subdivision surface> are unwanted variations along an intended straight extrusion: input cross-sections identical along a lattice direction acquire ripples in that direction under refinement. For a stationary arity-$m$ lattice rule $a_{i,j}$, extrusion along the first lattice coordinate is preserved exactly when $\sum_{i\equiv r\ (\mathrm{mod}\ m)}a_{i,j}$ is independent of the residue $r$ for every transverse offset $j$. Then summing along each row gives a single associated univariate <subdivision mask>. This direction-specific criterion does not promise invariance under every possible oblique extrusion.
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