Vector contours + vector hillshade (Mapterhorn DEM)

Contour line experiments.

Based on the maplibre-contour demo (makewithmaplibre.com/plugins/maplibre-contour). Both contour lines and the hillshade are produced in the browser, per tile, as vector geometry. Nothing is pre-rendered.

npm install
npm run dev

How it works

• maplibre-contour's DemSource fetches and decodes Mapterhorn terrarium tiles and caches them. It also feeds the optional raster hillshade layer used for comparison.
• A custom vterrain:// protocol (src/main.js) collects the 3×3 block of DEM tiles around each requested tile and passes it to a worker pool.
• src/terrain-worker.js resamples the DEM into a padded grid (overzoomed tiles are resampled to at least 256 px), then builds one vector tile with two layers:
  • contours: maplibre-contour's isoline tracer (src/contours.js), then src/contour-postprocess.js, which is where the proximity-based line edits go.
  • hillshade: Horn hillshade → blur → split into "shadow" and "highlight" tone fields → marching-squares polygons per threshold (d3-contour) → Douglas-Peucker → Chaikin → drop tiny rings (src/hillshade.js). Features have kind (shadow|highlight) and level. Shadow bands are nested, so a constant fill opacity stacks into darker tones.
• Shadow-density contours (contourMode: "density", the default) draw shadows with line density instead of fills (src/shadow-density.js). Contours are traced at a tight interval (denseInterval metres at z12, doubling per zoom level out). Every 4th line is kept everywhere. Every 2nd line is cut to areas at or above shadow class halfDensityFrom, and the remaining lines to class fullDensityFrom. The cut uses the same smoothed shadow field as the fill polygons.
• src/mvt.js is a small MVT encoder.

tileParams in src/main.js holds every setting that changes geometry. Editing one through the GUI bumps a version number in the tile URL, so MapLibre reloads the tiles. styleParams only change paint properties.

Notes from testing against real Mapterhorn tiles

• Tile URL, terrarium encoding and 512px tiles confirmed via https://tiles.mapterhorn.com/tilejson.json (which gives no maxzoom).
• Global coverage ends at z12. Some regions (Alps, Colorado) have tiles to z16+, but elsewhere z13+ returns 404, so DEM_MAXZOOM = 12. Using the deeper tiles would need a 404 fallback to the z12 parent.
• Shadow density at the default denseInterval: 20 looks good around z12. By z14, steep Alpine faces collapse into solid bands because the lines are less than 1px apart. Try 40–60 m, or handle it in src/contour-postprocess.js (proximity thinning).