nx_slice_plane.nx

nx_slice_plane.nx -- intersect a triangle mesh with a Z-plane. Returns a flat "segment soup" of 2D line segments (Q14 xy) where each segment is one triangle's intersection with the plane. This IS the slicer kernel. Bridges nx_mesh + nx_bvh + nx_polygon: - nx_bvh walk prunes triangles whose AABB doesn't straddle the plane (the slice-time win lever vs naive O(n_tris) scan) - per-candidate triangle, compute the 0/1/2 line segments the plane intersects - emit segments into a flat soup buffer Polygon assembly (linking endpoint-shared segments into closed contours) is a SEPARATE primitive (nx_slice_contour, P2.2) per Cardinal 9. Slice-plane is one job: mesh -> segment soup. Per cardinal NISHI_3D_PRINT_ROADMAP §2.7 (incremental slice): re-slicing on parameter changes with the same mesh reuses the same BVH; only this kernel re-runs. The win lever vs OrcaSlicer is: BVH-pruned candidates instead of full-mesh sweep, integer- only intersection arithmetic (no per-coord float epsilon drift). Triangle vs plane case analysis (signs s0, s1, s2 of vi.z - plane_z): all same sign (not zero) -> no intersection two same sign, one opposite -> one segment from the two straddled edges exactly one zero, others opposite -> one segment from the on-plane vertex to the straddled edge crossing exactly one zero, others same sign -> degenerate touch, skip exactly two zeros -> the triangle edge IS the segment (already 2D) all three zero -> coplanar tri, emit no segment (caller's slicer handles coplanar surfaces via a separate "horizontal face" pass) Coordinate convention: Q14 fixed-point inherited from nx_mesh. Plane-edge intersection (no overflow in i64 for slicer-scale coords):