[Shane98c]

Renders in map projections registered with MapLibre's experimental addProjection (maplibre-gl ≥ 6.15), such as EPSG:3031. Apps pass the layer the same projection objects in a new mapProjections option:

const antarctic: CRS = {
  name: 'EPSG:3031',
  projection: proj4('EPSG:4326', EPSG_3031_PROJ4),
  tileMatrixSet: { origin: [-4194304, 4194304], extentAtZoom0: 8388608 },
}
addProjection(antarctic)
map.setProjection({ type: 'EPSG:3031' })
map.addLayer(new ZarrLayer({ ...options, mapProjections: [antarctic] }))

How it works

• World mapping (src/[world-mapping.ts](https://github.com/tmp/cproj/src/world-mapping.ts)): computes positions in the projection's world square from its definition.
• Meshes (createWorldMesh): built directly in the world square and refined where the projection bends. Triangles that cross a seam or that the projection can't place are dropped.
• Region and level selection: works from each tile's footprint in the world square, including 0–360° data and far poles.
• Projection switches rebuild region geometry without refetching data.
• The layer hides and warns once if the map is in a registered projection it wasn't given.

Also in this PR

• Fixes a 1px gap where a tile edge lies exactly on a region edge, in all projections.
• Demo: Antarctic and Arctic polar stereographic, plate carrée, Equal Earth and CONUS Albers, over Natural Earth GeoJSON. Demo moves to maplibre-gl 6.16 and adds proj4.
• README: new "custom map projections" section.

Limitations

• MapLibre only. Mapbox doesn't draw custom layers outside mercator and globe.
• Basemap and terrain tiles must be in the same CRS as the map. The demo's DEM is Web Mercator, so terrain stays off in custom projections.

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Project	Deployment	Actions	Updated
zarr-layer	Ready	Preview	Nov 4, 2026 4:06am UTC