combine_polygons_below takes very long

[der-stefan]

As I'm currently preparing the OpenTopoMap vector tiles, I am playing around in depth with tilemaker's config and scripts. I tried to combine the landuse polygons (i.e. forests) on low zoom levels. However, tilemaker takes ages for this. In my case, germany-latest.osm.pbf takes ~10min when not using "combine_polygons_below" in the config. When enabled, I stopped tilemaker after ~2h.
Is this a know issue? Are you aware how the unification logic might be improved?

[systemed]

It is by definition a slow process and has been iteratively improved over tilemaker's life (e.g. #383), but that is quite an extreme slowdown and one I've not encountered before. It's always possible that there's a pathologically complex multipolygon which is boggling it.

If you could post your exact config that would help. I'd also suggest experimenting with simplification settings, because it's possible that it's not actually the combining that's the problem, it's trying to simplify the resulting polygon.

[der-stefan]

Thank you for your support. My exact configuration uses those two input files (https://github.com/der-stefan/OpenTopoMap/tree/master/vector/tilemaker) and this command:
tilemaker --config tilemaker-config-otm.json --process process-otm.lua --input data/germany-latest.osm.pbf --output data/germany.mbtiles --store tilemaker.store.d --shard-stores

[etienneJr]

@laem and I noticed the same thing for cartes.app tiles, so we ended up ... not using the combine_polygons_below option, but only the simplify_below options. Our lua and json files are here: https://codeberg.org/cartes/serveur/src/branch/master/tilemaker/resources

@der-stefan you can find some feedback on planet size tile generation here: #839 (comment) but since then @laem has scripted the automatic calculation every week, so he will probably have more feedback to share if you need.

[systemed]

#918 has a much faster combine_polygons_below implementation and might be worth trying.