First valuable feature: real terrain source for 3D Tiles quantized-mesh (#4493)

[clement-igonet]

Phase 2 of #4493, following the POC in #7952.

Converts quantized-mesh terrain tiles to terrain-RGB via addProtocol (same pattern as the geotiff plugin, per @HarelM's suggestion below), so the terrain source is a normal raster-dem source — map.setTerrain(), queryTerrainElevation(), draping, hillshade, and color-relief all work with zero core changes.

Bundles a single real Cesium World Terrain tile (Mont Blanc massif, z9) to keep the PR light; dynamic tile-covering resolution maps requested Mercator tiles onto it. The hardening follow-up (#7958) extends this to multi-zoom, live endpoints, and a worker.

Launch Checklist

• Confirm your changes do not include backports from Mapbox projects
• Briefly describe the changes in this PR.
• Link to related issues.
• Confirm you have read our AI policy here.

Assisted-By: Claude Sonnet 5 (claude-sonnet-5)

Summary by CodeRabbit

• New Features

  • Added an example demonstrating Cesium quantized-mesh terrain rendered as MapLibre terrain.
  • Added terrain tile processing with elevation queries, hillshade visualization, terrain exaggeration, and summit markers with popups.
  • Added a bundled Mont Blanc terrain tile asset for the example.

• Chores

  • Terrain files are now treated as binary assets to prevent unintended text modifications.

[HarelM]

The text was too long for me to read, but why not simply use addProtocol and convert the tiles' data into terrainRGB like the geotiff plugin does?

[HarelM]

Please make sure to read our AI policy. This is somewhat violating it and impacts my motivation to review the other PRs you opened...

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.25%. Comparing base (4435459) to head (89abeef).
⚠️ Report is 5 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7953   +/-   ##
=======================================
  Coverage   93.25%   93.25%           
=======================================
  Files         290      290           
  Lines       24650    24650           
  Branches     6471     6471           
=======================================
  Hits        22987    22987           
  Misses       1663     1663           

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[clement-igonet]

Please make sure to read our AI policy. This is somewhat violating it and impacts my motivation to review the other PRs you opened...

OK Guy !

[coderabbitai]

📝 Walkthrough

Walkthrough

Adds a bundled Mont Blanc quantized-mesh terrain tile and an example that decodes it, resamples it into Terrarium terrain-RGB tiles, and renders the result through MapLibre’s raster-dem terrain source.

Changes

Terrain source example

Layer / File(s)	Summary
Bundle terrain tile asset .gitattributes, docs/assets/examples/mont-blanc-terrain-tiles/9-531-386.terrain	Marks .terrain files as binary and adds the Mont Blanc terrain tile payload with metadata.
Decode and resample terrain test/examples/render-3d-tiles-terrain-source.html	Decodes quantized-mesh headers, vertices, and indices; interpolates elevations; handles coverage edges; and generates Terrarium terrain-raster data.
Connect terrain to MapLibre test/examples/render-3d-tiles-terrain-source.html	Registers the custom tile protocol, configures raster-dem terrain and hillshade, adds summit markers, and displays queried terrain elevation.

Estimated code review effort: 4 (Complex) | ~45 minutes

Sequence Diagram(s)

sequenceDiagram
  participant MapLibre
  participant quantizedMeshProtocol
  participant decodeQuantizedMesh
  participant buildTerrainRgbImageData
  participant RasterDemSource
  MapLibre->>quantizedMeshProtocol: request quantized-mesh://z/x/y
  quantizedMeshProtocol->>decodeQuantizedMesh: fetch bundled terrain payload
  decodeQuantizedMesh->>buildTerrainRgbImageData: decoded mesh
  buildTerrainRgbImageData->>RasterDemSource: ImageBitmap terrain-RGB tile
  RasterDemSource->>MapLibre: terrain elevation data

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🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly reflects the main change: adding a real terrain source for 3D Tiles quantized-mesh data.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[coderabbitai]

Actionable comments posted: 2

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@test/examples/render-3d-tiles-terrain-source.html`:
- Around line 328-335: Update findCoveringQMTiles to collect and return
intersecting tiles from every zoom in descending order instead of returning on
the first non-empty zoom. Preserve each tile’s computed bounds so
multiMeshElevationAt can select the deepest available mesh per point and use
lower-resolution parents for uncovered areas.
- Around line 357-375: The buildTerrainRgbImageData rasterization currently
performs synchronous per-pixel work without cancellation. Thread the protocol
handler’s AbortController signal through the tile fetch and rasterization path,
and make the row/pixel loops stop promptly when the signal is aborted; ensure
aborted work does not continue producing obsolete tile data.

🪄 Autofix (Beta)

Fix all unresolved CodeRabbit comments on this PR:

• Push a commit to this branch (recommended)
• Create a new PR with the fixes

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: d79f998f-e5c8-491d-a875-37fb5d598bc5

📥 Commits

Reviewing files that changed from the base of the PR and between e2c1f8d and f4158fc.

⛔ Files ignored due to path filters (1)

• docs/assets/examples/render-3d-tiles-terrain-source.png is excluded by !**/*.png

📒 Files selected for processing (3)

• .gitattributes
• docs/assets/examples/mont-blanc-terrain-tiles/9-531-386.terrain
• test/examples/render-3d-tiles-terrain-source.html