A Sparse Coarse Space for VBD from Element Eigenmodes

VBD’s per-vertex 3×3 solve already gives the locally optimal descent direction — there is no leverage left inside a single block. So why does it still slow down on problems with high stiffness contrast? The answer turns out to be a story about what basis you descend in, and once that picture is in place the fix — a sparse coarse correction built from per-element eigenmodes — almost suggests…