Combine libgeodesk and vtzero to build OpenStreetMap (OSM) vector tiles on demand from a GeoDesk Geographic Object Library (GOL).
Currently used to serve tiles for Ascend Maps.
On Linux, git clone https://github.com/styluslabs/geodesk-tiles, git submodule update --init, then make to create build/Release/server (compiler with C++20 support required), then run server <OSM GOL file> <ocean polygons GOL file> to provide a standard XYZ (WMTS) tile server at http://localhost:8080/tiles/{z}/{x}/{y}. The TCP port can be set with the --port option or iptables can be used to redirect port 80 to 8080 (so server needs no special permissions).
The GOL files can be created from OSM pbf files with the GeoDesk GOL utility. Daily prebuilt GOL files can be downloaded from OpenPlanetData.
To create a pbf for ocean polygons, simplified water polygons from https://osmdata.openstreetmap.de/data/water-polygons.html can be converted with ogr2osm. These are used for determining whether tiles without any coastline ways are ocean or land. A prebuilt oceans GOL file is available in releases.
Generated tiles are cached in an mbtiles file (i.e., an sqlite database containing map tiles), which can be set by the --db option (the default is planet.mbtiles).
Passing --build z/x/y to server will build the tile z/x/y and all children to z = 14, writing to planet.mbtiles, then exit.
The tile schema is defined by extending the TileBuilder class and implementing processFeature(), which is called for each feature intersecting the tile (as returned by a GeoDesk query). The TileBuilder class provides an API based on the Lua API used by tilemaker.
Call Find(<tag>) to read feature tags. The returned geodesk::TagValue object is convertible to bool (for existence testing), double (or int), and std::string. Id() returns the OSM id as a string, and IsClosed(), Length() (meters), and Area() (sq meters) return additional feature information. See AscendTileBuilder::WriteBoundary() for how to iterate over relation members (to be cleaned up).
MinZoom(z) returns <current tile z> >= z for use in determining if the current feature should be added to the tile.
Call Layer(<layer name>) or LayerAsCentroid(<layer name>) to add the current feature (or its centroid as a single point) to the tile, then call Attribute(<key>, <value>) and AttributeNumeric(<key>, <value>) to add attributes.
Coastline ways (natural=coastline) should be passed to addCoastline() instead of being added to tile. After all features are processed, ocean polygons will be generated and processFeature() called with no feature set - in this case it should call Layer() and Attribute() as appropriate for ocean features.
For low zoom tiles, the features to be processed should be limited by populating TileBuilder::m_queries with a list of GeoDesk queries (which should reference only indexed keys).
ascendtiles.cpp (a direct translation of the tilemaker script process.lua) implements the Ascend Maps schema, which mostly just uses unmodified OSM tags for feature attributes. Adapting for other schemas should be relatively straightforward.
Rebuilding (i.e., running make) after schema changes should only take a few seconds.
server.cpp uses cpp-httplib to provide an HTTP server and sqlite to save generated tiles to an mbtiles file. Tile builder threads (number set by --threads option, defaulting to CPU cores minus one) share a queue to prevent duplicate work.
Simple Cohen–Sutherland clipping is used in clipper.h; more robust clipping for edge cases should be added in the future.
For simplicity, every tile is built independently - processed geometry and attributes are not reused for building parents or children of tile. Also, geometry with identical attributes is not currently combined in tile.
Results for dense urban area (San Francisco): time to process tile is ~1x to ~3x time to gzip tile (miniz level 5).
Tile 2617/6332/14/14 (243209 bytes) built in 43.5 ms (22.3 ms process 8486/10451 features w/ 116260 points, 21.2 ms gzip 489316 bytes)
Tile 1308/3166/13/13 (50387 bytes) built in 17.8 ms (14.5 ms process 2856/30380 features w/ 10158 points, 3.3 ms gzip 105501 bytes)
Tile 654/1583/12/12 (208908 bytes) built in 76.2 ms (56.4 ms process 14587/151092 features w/ 41012 points, 19.8 ms gzip 484240 bytes)
Tile 327/791/11/11 (203717 bytes) built in 93.0 ms (69.6 ms process 17917/306543 features w/ 58383 points, 23.5 ms gzip 494058 bytes)
Tile 163/395/10/10 (215077 bytes) built in 91.4 ms (72.1 ms process 19622/419999 features w/ 68507 points, 19.3 ms gzip 531326 bytes)