contour_cells

Whether each cell of a grid of samples straddles the surface: its eight corner samples hold both a value below zero and one at or above it. It reads vals, the distance at every grid sample (x fastest), over the grid the params state (sx, sy samples along x and y; nx, ny, nz cells; ox, oy, oz the position of sample (0, 0, 0); h the cell size). flag[c] is 1 for a straddling cell and 0 otherwise, the keep mask compact turns into the list of cells that own a vertex. The sdf.mesh node runs it first.

Dispatch over the cells. Exact: each flag has one writer.