coarsen

Fast, multithreaded drop-ins for shapely.coverage_simplify and shapely.simplify, with byte-identical output to GEOS 3.13.1. Pure Rust ports of GEOS's CoverageSimplifier and TopologyPreservingSimplifier behind a thin Shapely wrapper. The Rust core links neither GEOS nor PROJ and performs no dynamic library loading.

pip install coarsen

import coarsen
import shapely

polygons = [shapely.box(0, 0, 10, 10), shapely.box(10, 0, 20, 10)]
result = coarsen.coverage_simplify(polygons, 5.0, threads=8)  # shapely.coverage_simplify
lines = coarsen.simplify(polygons, 2.0)  # shapely.simplify

simplify(geometry, tolerance, preserve_topology=True, *, threads=None, **kwargs) matches shapely.simplify: any geometry type, broadcasting, scalar in and scalar out, None passthrough, and the out and where ufunc keywords. The default topology-preserving mode runs in Rust. preserve_topology=False calls shapely.simplify, because GEOS repairs Douglas-Peucker polygons with buffer(0).

coverage_simplify(geometry, tolerance, *, simplify_boundary=True, threads=None) returns a NumPy object array with the input shape; scalar geometry input returns a scalar geometry, matching Shapely. simplify_boundary=False implements GEOS simplifyInner. threads=None uses Rayon's default pool; a positive integer creates a pool for that call. Rust parses, simplifies, and writes WKB without the GIL. Tolerance uses input coordinate units; project longitude/latitude data before using a tolerance in metres. Requires Python ≥3.10, NumPy, and Shapely ≥2.1.

The parity target is GEOS 3.13.1, bundled in Shapely 2.1.2's reference wheels. Tests compare WKB bytes without normalization at one and eight threads, including both boundary modes, the original 174 fixtures, real UTM parcels, Voronoi cells, grids, holes, MultiPolygons, empty input, zero tolerance, and Z/M coordinates. Z/M rebuilding follows GEOS's quirks. Other GEOS versions can produce different results. Parity is verified on these cases, not a proof for every floating-point input. Nonfinite XY coordinates are rejected. Invalid coverages are not repaired. The Rust library retains coverage validation; no Python invalid-edges helper is exposed because the port computes an invalid-polygon mask rather than edge geometry.

simplify is tested the same way: WKB bytes against Shapely 2.1.2 on 17,920 seeded random geometries of every type and 192 real parcels, at one and eight threads.

Coverage simplification, tolerance 5 m, median of three runs:

simplify (topology-preserving), single run on a shared login node:

Measured timings cover the complete public API, including WKB conversion, at 5 m in each tile's UTM CRS. See benchmark details.

LGPL-2.1-or-later. Derived from GEOS work by Martin Davis, Paul Ramsey, and other contributors; see NOTICE and LICENSE. Build, test, and release instructions are in CONTRIBUTING.md.