fill-extrusion-height seems incorrect with terrain

[geobuilding]

• Mapbox GL JS: version 2.8.2

Without terrain https://jsbin.com/kizurex/edit?js,output

With terrain https://jsbin.com/vibebum/edit?js,output

[ubukawa]

Hello. Thank you mapbox gl js colleagues for your wonderful development.

I am a newbie of terrain 3D, and I have a similar issue of fill-extrusion-height with 3D terrain. So, let me report my case as well.

With my internal data, I observed an issue of fill extrusion height with 3D terrain. (There was no issue without 3D terrain.) At the tile border, fill-extrusion-height of some buildings are much higher than expected ones.

I cannot open my internal data, so I have prepare some test in the public environment as below.

• Mapbox GL JS: version 2.8.2

• Map:

  • with Terrain https://ubukawa.github.io/elbox/nyc-no-terrain.html (style: https://ubukawa.github.io/elbox/nyc-Terrain.json)
  • without Terrain 3D https://ubukawa.github.io/elbox/nyc-terrain.html (style: https://ubukawa.github.io/elbox/nyc-noTerrain.json)

• Vector tile source (I converted it from OSM data using tippecanoe)

  • URL: https://ubukawa.github.io/osm-neighbor/osm-ny/{z}/{x}/{y}.pbf
  • Zoom level: 5-13
  • Area: around New York

• Terrain Source (I converted it from 30 meter DEM using mapbox/rio-rgbify)

  • URL: https://ubukawa.github.io/AW3D30-RGB-NY/zxy/{z}/{x}/{y}.png
  • Zoom level: 4-12
  • Tile size: 256

I know that the terrain height is added to fill-exrusion-height, but I think there are some issues at around the tile border.

At the tile border (for both vector tile source and terrain source): Some buildings are taller than expected. Left (no terrain 3D): https://ubukawa.github.io/elbox/nyc-no-terrain.html#16/40.784721/-74.003817/-38.1/53 Right (with terrain 3D) : https://ubukawa.github.io/elbox/nyc-terrain.html#16/40.784721/-74.003817/-38.1/53

Left (no terrain 3D): https://ubukawa.github.io/elbox/nyc-no-terrain.html#15.64/40.814385/-73.936267/0/35 Right (with terrain 3D) : https://ubukawa.github.io/elbox/nyc-terrain.html#15.64/40.814385/-73.936267/0/35

At the tile border (tile border for vector tile but not tile border for terrain source): Some buildings are lower than expected.

Left (no terrain 3D): https://ubukawa.github.io/elbox/nyc-no-terrain.html#15.99/40.809697/-73.960373/0/46 Right (with terrain 3D) : https://ubukawa.github.io/elbox/nyc-terrain.html#15.99/40.809697/-73.960373/0/46

I would appreciate it if someone could advise me how I can avoid these issues. Thank you!!