Contour

Topographic level lines: a seeded heightfield cut at evenly spaced levels, the terrain-map look as pure line art.

What it is

Where flowfield draws a noise field’s gradient as streamlines, this draws its level sets: fractal value noise sampled on a grid, marching squares at each iso-level, segments chained into contour lines — nested closed loops around the peaks and hollows, open lines running off the page edge.

The implementation’s guarantees

• Interior contours close, always: an open contour that dangles mid-page is a chaining bug, and the test says so directly. Chaining is by exact endpoint equality (marching squares interpolates shared edges to identical points), not tolerance fudging.
• Tiny summit rings are culled, and the cull is reported: a 4 mm² peak ring fails the colourability area floor and survives every level-thinning attempt — the same peak just gets re-cut — so closed loops under the floor are dropped at compose time and counted in culled_below_area_floor.
• The escalation lever is level count (fewer isolines, more white space); the noise is self-contained value noise, seeded and octaved, because generator crates never share code.
• Rating basis: none — a design. Honesty fields: contour counts (closed vs open-at-edge), levels, cull count, attempt.