Allow negative fill-extrusion-base and fill-extrusion-height (#8051)

[clement-igonet]

Fill-extrusion layers can now extrude below ground level: negative fill-extrusion-base and fill-extrusion-height values render underground volumes instead of being clamped to the ground. This enables underground floor levels for indoor mapping, for example the basement levels of a station or a building (#8051).

Before / after, an above/below-ground floor stack (levels +2 to -3), left is current main, right is this PR:

Live comparison: https://maplibre.confinia.io/debug/underground-compare-8051.html

Pairs with maplibre/maplibre-style-spec#1794, which lifts the minimum: 0 constraint on both properties.

Launch Checklist

• Confirm your changes do not include backports from Mapbox projects (unless with compliant license) - if you are not sure about this, please ask!
• Briefly describe the changes in this PR.
• Link to related issues: fill-extrusion-base cannot go below 0 #8051
• Include before/after visuals or gifs if this PR includes visual changes.
• Write tests for all new functionality.
• Post benchmark scores.
• Add an entry to CHANGELOG.md under the ## main section.
• Confirm you have read our AI policy here.

Assisted-By: Claude (claude-fable-5)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.92%. Comparing base (37e08c1) to head (9168595).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

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  Coverage   93.92%   93.92%           
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  Files         290      290           
  Lines       24916    24916           
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  Hits        23402    23402           
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[CommanderStorm]

I think the one on the left looks quite buggy.
If negative extrusion meant hole, that would make more sense to me.
Just a polygon at ground level is something that an average user likely does not expect when they set this negative.

[HarelM]

Can you make the image use something more transparent when the building is underground maybe?
The example above is just a bit confusing because the OSM raster is not really "cut" at the right place.
I would expect a hole for something that's only negative maybe.
Bottom line, I'm not sure it's currently well defined from a user appearance perspective. While the spec itself may support negative values, it's not clear what the user should see when from the provided image.

[clement-igonet]

Good point, reworked the visual. The PR only defines the geometry; what the user sees is up to the style, and negative bases make an excavation pattern possible: a ring polygon extruded from 0 down forms the pit walls, open in the middle, with the floors inside (right side; on main everything collapses to ground level). A real hole in the draped ground would need polygon clipping of the basemap, which I think is a separate feature. The ground raster is dimmed in the demo for visibility, and an opaque ground-level cover around the pit occludes the interior at the rim, which is what makes it read as a hole rather than a platform.

Live version: https://maplibre.confinia.io/debug/underground-pit-8051.html

[clement-igonet]

Tried your suggestion and it turns out the transparent-ground appearance works purely at the style level: draw the basemap raster, then the underground layer, then the same raster again with raster-opacity around 0.5, then the above-ground layers. Depth ordering makes the semi-transparent ground draw above everything below elevation 0, so underground volumes ghost through it while above-ground buildings stay crisp (right side; left is main). Over normal ground the second raster is invisible, so no masking is needed.

Live: https://maplibre.confinia.io/debug/underground-xray-8051.html