kopia lustrzana https://github.com/onthegomap/planetiler
125 wiersze
7.6 KiB
Markdown
125 wiersze
7.6 KiB
Markdown
# Planetiler Architecture
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Planetiler builds a map in 3 phases:
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1. [Process Input Files](#1-process-input-files) according to
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the [Profile](planetiler-core/src/main/java/com/onthegomap/planetiler/Profile.java) and write rendered tile features
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to intermediate files on disk
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2. [Sort Features](#2-sort-features) by tile ID
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3. [Emit Vector Tiles](#3-emit-vector-tiles) by iterating through sorted features to group by tile ID, encoding, and
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writing to the output MBTiles file
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## 1) Process Input Files
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First, Planetiler
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reads [SourceFeatures](planetiler-core/src/main/java/com/onthegomap/planetiler/reader/SourceFeature.java)
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from each input source:
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- For "simple
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sources" [NaturalEarthReader](planetiler-core/src/main/java/com/onthegomap/planetiler/reader/NaturalEarthReader.java)
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or [ShapefileReader](planetiler-core/src/main/java/com/onthegomap/planetiler/reader/ShapefileReader.java) can get the
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latitude/longitude geometry directly from each feature
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- For OpenStreetMap `.osm.pbf` files,
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[OsmReader](planetiler-core/src/main/java/com/onthegomap/planetiler/reader/osm/OsmReader.java)
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needs to make 2 passes through the input file to construct feature geometries:
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- pass 1:
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- nodes: store node latitude/longitude locations in-memory or on disk
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using [LongLongMap](planetiler-core/src/main/java/com/onthegomap/planetiler/collection/LongLongMap.java)
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- ways: nothing
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- relations: call `preprocessOsmRelation` on the profile and store information returned for each relation of
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interest, along with relation member IDs in-memory using
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a [LongLongMultimap](planetiler-core/src/main/java/com/onthegomap/planetiler/collection/LongLongMultimap.java).
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- pass 2:
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- nodes: emit a point source feature
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- ways:
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- lookup the latitude/longitude for each node ID to get the way geometry and relations that the way is contained
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in
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- emit a source feature with the reconstructed geometry which can either be a line or polygon, depending on
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the `area` tag and whether the way is closed
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- if this way is part of a multipolygon, also save the way geometry in-memory for later in
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a [LongLongMultimap](planetiler-core/src/main/java/com/onthegomap/planetiler/collection/LongLongMultimap.java)
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- relations: for any multipolygon relation, fetch the member geometries and attempt to reconstruct the multipolygon
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geometry
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using [OsmMultipolygon](planetiler-core/src/main/java/com/onthegomap/planetiler/reader/osm/OsmMultipolygon.java),
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then emit a polygon source feature with the reconstructed geometry if successful
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Then, for each [SourceFeature](planetiler-core/src/main/java/com/onthegomap/planetiler/reader/SourceFeature.java),
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render vector tile features according to
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the [Profile](planetiler-core/src/main/java/com/onthegomap/planetiler/Profile.java) in a worker thread (default 1 per
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core):
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- Call `processFeature` method on the profile for each source feature
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- For every vector tile feature added to
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the [FeatureCollector](planetiler-core/src/main/java/com/onthegomap/planetiler/FeatureCollector.java):
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- Call [FeatureRenderer#accept](planetiler-core/src/main/java/com/onthegomap/planetiler/render/FeatureRenderer.java)
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which for each zoom level the feature appears in:
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- Scale the geometry to that zoom level
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- Simplify it in screen pixel coordinates
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- Use [TiledGeometry](planetiler-core/src/main/java/com/onthegomap/planetiler/render/TiledGeometry.java)
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to slice the geometry into subcomponents that appear in every tile it touches using the stripe clipping algorithm
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derived from [geojson-vt](https://github.com/mapbox/geojson-vt):
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- `sliceX` splits the geometry into vertical slices for each "column" representing the X coordinate of a vector
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tile
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- `sliceY` splits each "column" into "rows" representing the Y coordinate of a vector tile
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- Uses an [IntRangeSet](planetiler-core/src/main/java/com/onthegomap/planetiler/collection/IntRangeSet.java) to
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optimize processing for large filled areas (like oceans)
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- If any features wrapped past -180 or 180 degrees longitude, repeat with a 360 or -360 degree offset
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- Reassemble each vector tile geometry and round to tile precision (4096x4096)
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- For
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polygons, [GeoUtils#snapAndFixPolygon](planetiler-core/src/main/java/com/onthegomap/planetiler/geo/GeoUtils.java)
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uses [JTS](https://github.com/locationtech/jts) utilities to fix any topology errors (i.e. self-intersections)
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introduced by rounding. This is very expensive, but necessary since clients
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like [MapLibre GL JS](https://github.com/maplibre/maplibre-gl-js) produce rendering artifacts for invalid
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polygons.
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- Encode the feature into compact binary format
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using [FeatureGroup#newRenderedFeatureEncoder](planetiler-core/src/main/java/com/onthegomap/planetiler/collection/FeatureGroup.java)
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consisting of a sortable 64-bit `long` key (zoom, x, y, layer, sort order) and a binary value encoded
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using [MessagePack](https://msgpack.org/) (feature group/limit, feature ID, geometry type, tags, geometry)
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- Add the encoded feature to
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a [WorkQueue](planetiler-core/src/main/java/com/onthegomap/planetiler/worker/WorkQueue.java)
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Finally, a single-threaded writer reads encoded features off of the work queue and writes them to disk
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using [ExternalMergeSort#add](planetiler-core/src/main/java/com/onthegomap/planetiler/collection/ExternalMergeSort.java)
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- Write features to a "chunk" file until that file hits a size limit (i.e. 1GB) then start writing to a new file
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## 2) Sort Features
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[ExternalMergeSort](planetiler-core/src/main/java/com/onthegomap/planetiler/collection/ExternalMergeSort.java) sorts all
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of the intermediate features using a worker thread per core:
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- Read each "chunk" file into memory
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- Sort the features it contains by 64-bit `long` key
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- Write the chunk back to disk
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## 3) Emit Vector Tiles
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[MbtilesWriter](planetiler-core/src/main/java/com/onthegomap/planetiler/mbtiles/MbtilesWriter.java) is the main driver.
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First, a single-threaded reader reads features from disk:
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- [ExternalMergeSort](planetiler-core/src/main/java/com/onthegomap/planetiler/collection/ExternalMergeSort.java) emits
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sorted features by doing a k-way merge using a priority queue of iterators reading from each sorted chunk
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- [FeatureGroup](planetiler-core/src/main/java/com/onthegomap/planetiler/collection/FeatureGroup.java) collects
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consecutive features in the same tile into a `TileFeatures` instance, dropping features in the same group over the
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grouping limit to limit point label density
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- Then [MbtilesWriter](planetiler-core/src/main/java/com/onthegomap/planetiler/mbtiles/MbtilesWriter.java) groups tiles
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into variable-sized batches for workers to process (complex tiles get their own batch to ensure workers stay busy
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while the writer thread waits for finished tiles in order)
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Then, process tile batches in worker threads (default 1 per core):
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- For each tile in the batch, first check to see if it has the same contents as the previous tile to avoid re-encoding
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the same thing for large filled areas (i.e. oceans)
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- Encode using [VectorTile](planetiler-core/src/main/java/com/onthegomap/planetiler/VectorTile.java)
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- gzip each encoded tile
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- Pass the batch of encoded vector tiles to the writer thread
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Finally, a single-threaded writer writes encoded vector tiles to the output MBTiles file:
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- Create the largest prepared statement supported by SQLite (999 parameters)
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- Iterate through finished vector tile batches until the prepared statement is full, flush to disk, then repeat
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- Then flush any remaining tiles at the end
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