contour_crossings

Where the surface crosses each of the twelve edges of every straddling cell: the point (cross_p) and whether it crosses (cross_w, 1 or 0). The crossing is the linear interpolation of the two corner samples to zero. An edge that does not cross carries its cell's centre, a point any shape can be read at. Slot s of the outputs is the s-th straddling cell, cells[s], and edge e of it is slot s * 12 + e. It reads vals and the grid params as contour_cells does, cells (the list compact makes of the straddling cells) and domain_count (the live count compact made, bound in every variant): a slot at or past it holds the point (0, 0, 0) and weight 0. contour_solve reads the outputs.

Dispatch over the straddling cells, or the capacity of their list. Exact: each slot has one writer.