Extrusion invariance of Loop subdivision
= Extrusion invariance of Loop subdivision
On an infinite regular triangular grid, height data constant along one edge direction remain constant along that direction under <Loop subdivision>. Across rows the induced rule is $g'_{2j}=(g_{j-1}+6g_j+g_{j+1})/8$, $g'_{2j+1}=(g_j+g_{j+1})/2$, the centered <Cardinal cubic B-spline> rule.
// Target: analysis.bigb