EXPERIMENTAL support for arbitrary JSON objects as feature attributes

[e-n-f]

Prototype for mapbox/vector-tile-spec#75

To test:

git checkout object-attributes
make clean
make -j
./tippecanoe -z0 -f -o compound.mbtiles tests/object/in.json
./tippecanoe-decode compound.mbtiles

and note the properties in the output that contain JSON objects rather than just stringified representations of them.

[clarkcorey]

Is this something that you're considering moving out of experimental and into master? I feel like this is super useful, and would imagine the community would find it handy as well. It would give the tile data a lot of flexibility - both on the client and in the stylesheet. Thanks.

[e-n-f]

Unfortunately this is not the format for compound attributes that has made it into the work for version 3 of the vector tile spec, so this particular branch is basically a dead end.

[stdmn]

@ericfischer Any sense of when/whether something like this will be added to Tippecanoe soon? It looks like the option of parsing the tiles in browser was shot down by the MB team and it looks like there is an accepted commit to implement an 'in' expression. Unfortunately, without a way to encode arrays in the feature properties in Vector tiles, the 'in' expression is essentially worthless.

I know this is a large project but wanted to see if there is any movement.

[e-n-f]

There is no movement that I am aware of. Vector tile spec version 3 may yet be a thing (probably in the form in https://github.com/mapbox/vector-tile-spec/tree/v3.0-development/3.0, not the one that this PR writes), but it requires a whole lot of projects to move simultaneously, most of which have other priorities. I don't know where GL-JS is getting these features that have arrays for attributes, other than maybe from GeoJSON.