contour_quads

One quad per grid edge the surface crosses, spanning the four cells around it. Edge e is the edge of sample e / 3 along axis e % 3. quad_flag[e] is 1 for an edge that owns a quad and quad_v[e] holds its four vertex numbers, wound so every face points away from the solid. An edge on the grid's boundary, or one with a cell around it that owns no vertex, owns no quad.

vertex_of[c] is the number of the vertex cell c owns, or below 0 for a cell that owns none: the inverse map compact writes beside the list of straddling cells. It reads vals and the grid params as contour_cells does. The sdf.mesh node compacts quad_flag into the kept quads contour_triangles splits.

Dispatch over 3 * samples edges. Exact: each edge has one writer.