coarsen

Parallel geometry simplification for Shapely, powered by Rust. Reduce polygon and line vertices, preserve topology, and keep shared polygon boundaries aligned. Useful for parcel maps, land-cover polygons, and large vector datasets.

Documentation · API · Benchmarks · Contributing · Security

Install

pip install coarsen

Requires Python ≥3.10, NumPy, and Shapely ≥2.1. The Rust core links neither GEOS nor PROJ; the Python interface uses Shapely for geometry conversion. Building from source requires Rust ≥1.85 and maturin.

Simplify shared boundaries

Use coverage_simplify for a polygon coverage: polygons with non-overlapping interiors and exactly matching shared edges. All input polygons are processed together so their simplified boundaries remain aligned.

import coarsen
import shapely

polygons = [
    shapely.Polygon([(0, 0), (5, 0), (5.1, 2), (5, 5), (0, 5)]),
    shapely.Polygon([(5, 0), (10, 0), (10, 5), (5, 5), (5.1, 2)]),
]
result = coarsen.coverage_simplify(polygons, 1.0, threads=8)

# Keep the outer coverage boundary unchanged.
inner_only = coarsen.coverage_simplify(polygons, 1.0, simplify_boundary=False)

Tolerance uses coordinate units. Project longitude/latitude data to a suitable projected CRS before using a tolerance in metres. Invalid coverages are not repaired; check them with shapely.coverage_is_valid when validity is uncertain.

Simplify individual geometries

line = shapely.LineString([(0, 0), (1, 0.1), (2, 0)])
result = coarsen.simplify(line, 0.2)
assert list(result.coords) == [(0.0, 0.0), (2.0, 0.0)]

simplify preserves each geometry's topology by default. It supports broadcasting, None, and out/where; it does not preserve shared boundaries between separate array elements. preserve_topology=False delegates to Shapely. See the API reference for compatibility details and threading guidance.

Performance and compatibility

The recorded coverage benchmark processes 260,200 polygons across three projected FTW tiles. Eight threads took 6.42–9.65 s per tile, versus 92.03–94.71 s for Shapely, a 9.5–14.5× speedup on that workload. These are historical shared-node measurements, not a guarantee for other hardware or geometry distributions. Methodology, raw samples, and limitations.

The algorithms port GEOS 3.13.1. Tests compare unnormalized WKB bytes against Shapely 2.1.2 at one and eight threads using original GEOS fixtures, seeded random geometries, and real projected parcels. Parity is established for the tested cases; other GEOS versions and untested inputs can differ. Z/M behavior follows the reference algorithm and may drop dimensions. Nonfinite coordinates used in indexes are rejected.

Development

make install
make check
make docs-serve

Requires uv and Rust. make check runs Rust formatting, Clippy, Rust and Python tests, Ruff, ty, and the strict documentation build. Run make build after editing Rust. See CONTRIBUTING.md for release and platform notes.

License and attribution

LGPL-2.1-or-later. Derived from GEOS work by Martin Davis, Paul Ramsey, and other contributors; see NOTICE.