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3D bridges

A road on 3D terrain is draped: painted onto the terrain, so it follows every bump the elevation data has. Right for a road on the ground, wrong for one on a bridge - the deck sags into the valley it crosses, and a tunnel climbs over the hill it goes through. With 3D bridges on, a feature the style marks as a span is laid straight between its two ends instead, on the ground it leaves and the ground it reaches, and a bridge deck stands as a thin extrusion carrying its road, its labels, its one-way arrows.

Off by default. Nothing runs until an app turns it on, and a map that never does pays nothing.

Turning it on​

Object API:

terrainOptions.setBridges3DEnabled(true);

Facade:

map.set("terrain.bridges3DEnabled", true);

Needs 3D terrain: with the terrain off, or while the map is auto-flattened, spans drape like the ground and decks are not drawn.

What the style says​

Three properties, one per symbolizer, each drape (the default), span or underground:

PropertyOnEffect with the option on
line-elevation-mode: spana lineThe line runs straight between its two ends, at the ground height of each. A road on a bridge, a rail line, a footbridge.
polygon-elevation-mode: spana fillThe fill lies on the same straight surface: a bridge bed.
building-elevation-mode: spanan extrusionA deck: building-min-height and building-height become a thickness measured from the road surface, negative values hanging below it. The roof wears the span lines drawn on it.
*-elevation-mode: undergroundanyParses, and drapes like the ground for now - a tunnel drawn on the surface in its tunnel style, as mapbox does.

A converted Mapbox Standard carries the tags the tiler needs (structure = 'bridge'), so the rules are one line each. The demo's deck:

#structure[class = 'land']['mapnik::geometry_type' = 3]::land_structure_deck {
  building-fill: #98a0b0;
  building-elevation-mode: 'span';
  building-min-height: -1.5;   /* a 1.4 m slab... */
  building-height: -0.1;       /* ...0.1 m under the road surface */
}

mapbox2css does not emit these properties yet; add them to the converted style by hand.

What you get​

  • A deck that is one flat plane between its abutments whatever the elevation data does under it, whatever tile cut it, and through the auto-flatten ramp. Its road is drawn on the roof, its labels and arrows stand on it, it casts a shadow.
  • Where the deck's outline runs on past the road's bridge segment - onto the quay - the roof shows the ground's own roads and crosswalks, so the approach is continuous.
  • A bridge opened mid-deck, with neither end on screen, still finds its ends: the SDK fetches the coarser tiles that hold them.

What it costs​

On: two elevation reads per bridge per cull, a small render-to-texture per tile that holds a deck, and up to 32 coarse tiles fetched for bridges whose ends are off screen. Measured at Grenoble: 3.5-6.8 ms of union work per second of panning.

Off: none of that. The switch is checked before any span work starts; the only trace is the per-feature "is this a span" test the renderer already made.

Known limits​

The two sides of an abutment are rarely at one height, and a level deck shows a sliver of ground through one corner or a gap under the other; a corner that stops short of the road's bridge segment leaves a few metres of road on the ground. The mechanism, the history and the open list are in the renderer page.