polygol

Boolean polygon clipping/overlay operations (union, intersection, difference, xor) on Polygon and MultiPolygon geometry. A Go port of the mfogel/polygon-clipping JS library.

Quickstart

A := [][][][]float64{{{{0.0, 0.0}, {0.0, 6.0}, {6.0, 6.0}, {6.0, 0.0}, {0.0, 0.0}}}}
B := [][][][]float64{{{{5.0, 1.0}, {5.0, 5.0}, {7.0, 5.0}, {7.0, 1.0}, {5.0, 1.0}}}}
C := [][][][]float64{{{{3.0, -2.0}, {3.0, 2.0}, {9.0, 2.0}, {9.0, -2.0}, {3.0, -2.0}}}}

union, _ := polygol.Union(A, B, C)
intersection, _ := polygol.Intersection(A, B, C)
difference, _ := polygol.Difference(A, B, C)
xor, _ := polygol.XOR(A, B, C)

API

polygol only has a single type Geom which represents the coordinate structure of a MultiPolygon:

type Geom [][][][]float64

Each Boolean operation (union, intersection, difference and XOR) takes in one subject Geom with any number of clipping Geom's and then returns a single result Geom:

func polygol.Union(geom polygol.Geom, moreGeoms ...polygol.Geom) (polygol.Geom, error)

Note that particularly large geometries may cause errors that will suggest increasing environment variables POLYGOL_MAX_QUEUE_SIZE and POLYGOL_MAX_SWEEPLINE_SEGMENTS.

Examples

The examples page includes some information on how polygol can interface with Go geometry libraries like paulmach/go.geojson, paulmach/orb and twpayne/go-geom.

Test coverage

At the moment, polygol aims to have 100% test coverage relative to polygon-clipping; all unit and end-to-end tests have been ported over to Go.

Dependencies

polygol only has a single dependency: engelsjk/splay-tree. This is a Go port of the JS library w8r/splay-tree which is used in polygon-clipping. BST splay trees are used for the coordinate rounder, the sweep event priority queue and the sweep line itself.

Why `polygon-clipping`?

The polygon-clipping library is a well-tested implementation of the Martínez-Rueda-Feito algorithm and is also currently used behind-the-scenes in both TurfJS and the OpenStreetMap iD editor. Naively porting this library from JS to Go is intended to provide a fairly robust polygon clipping capability in Go without the need for GEOS bindings.

Improvements

Future improvements may include algorithm restructuring and built-in interfaces to common Go geometry libraries.

Performance

The Martínez-Rueda-Feito polygon clipping algorithm computes the Boolean operations in O((n+k)*log(n)) time, where n is the total number of edges of all polygons and k is the number of intersections between edges.

References

The algorithm implemented here and in polygon-clipping is based on the following paper:

A new algorithm for computing Boolean operations on polygons by Francisco Martínez, Antonio J. Rueda, Francisco R. Feito (2009)

Additional information can also be found in the follow-up paper:

A simple algorithm for Boolean operations on polygons by Francisco Martínez, Carlos Ogayar, Juan R. Jiménez and Antonio J. Rueda (2013)

Implementation survey of the Martínez-Rueda-Feito algorithm

Language	URL	Notes
C++	fmartin/bool_op	original implementation
JavaScript	w8r/martinez	an extension of the original algorithm
JavaScript	mfogel/polygon-clipping	originally forked from w8r/martinez, notably used here and here
JavaScript	mapbox/polyclip	by mourner! archived though
JavaScript	velipso/polybooljs	"based somewhat" on the original
Go	engelsjk/polygol	this one, a port of mfogel/polygon-clipping
Go	toanqng/martinez-rueda	based on kudm761/martinez-rueda-php
Go	akavel/polyclip-go	"known to have bugs"
Rust	21re/rust-geo-booleanop	closely follows w8r/martinez
Python	lycantropos/martinez	port of the original
Python	lycantropos/clipping
PHP	kudm761/martinez-rueda-php	based on the original
C/ActionScript3	akavel/martinez-src	copy of the original with an AS3 port
Scala	JonMcPherson/martinez-polygon-clipper	ported from w8r/martinez

TheFuzzyGiggler/PolyGOn