contour_triangles

Each kept quad as two triangles: pt holds, per quad, the vertex numbers of its corners (a b c, a c d), and vstart the position of each triangle's first corner in pt. Slot n of the dispatch is the n-th quad edges[n] names (the list compact makes of quad_flag), read from quad_v of contour_quads, so the quads are numbered in edge order.

Dispatch over the kept quads. The sdf.mesh node dispatches over every grid edge and states the kept count times 6 as the vertex count and times 2 as the primitive count, so the slots past the kept quads lie past the live elements. Exact: each slot has one writer.