Outline building parts

[1ec5]

This is a 2D style, not a 3D style, but nonetheless 3D building parts can help make sense of a building whose footprint is otherwise an unrecognizable blob. City guide maps often reserve this treatment for the most iconic buildings in the city and otherwise color-code building footprints by building use. However, OpenMapTiles doesn’t expose anything that would allow us to distinguish iconic buildings or building uses, so we might as well give buildings some texture instead.

OpenMapTiles includes building parts in the building layer with hide_3d set to true. The buildings are already being rendered using a fixed-height fill extrusion layer, to simulate a drop shadow. Building parts can simply be slightly taller than building outlines but remain fixed-height. To avoid the information overload described in gravitystorm/openstreetmap-carto#1857 (comment), we can limit building parts to the highest zoom levels and stick them in the same layer as the building footprints, which will prevent overlapping parts and outlines from getting an additive fill.