Polygon-clipping NPM

polygon-clipping

Apply boolean Polygon clipping operations (intersection, union, difference, xor) to your Polygons & MultiPolygons.

Quickstart

const polygonClipping = require('polygon-clipping')

const poly1 = [[[0,0],[2,0],[0,2],[0,0]]]
const poly2 = [[[-1,0],[1,0],[0,1],[-1,0]]]

polygonClipping.union       (poly1, poly2 /* , poly3, ... */)
polygonClipping.intersection(poly1, poly2 /* , poly3, ... */)
polygonClipping.xor         (poly1, poly2 /* , poly3, ... */)
polygonClipping.difference  (poly1, poly2 /* , poly3, ... */)

API

/* All functions take one or more [multi]polygon(s) as input */

polygonClipping.union       (<geom>, ...<geoms>)
polygonClipping.intersection(<geom>, ...<geoms>)
polygonClipping.xor         (<geom>, ...<geoms>)

/* The clipGeoms will be subtracted from the subjectGeom */
polygonClipping.difference(<subjectGeom>, ...<clipGeoms>)

Input

Each positional argument (<geom>) may be either a Polygon or a MultiPolygon. The GeoJSON spec is followed, with the following notes/modifications:

MultiPolygons may contain touching or overlapping Polygons.
rings are not required to be self-closing.
rings may contain repeated points, which are ignored.
rings may be self-touching and/or self-crossing. Self-crossing rings will be interpreted using the non-zero rule.
winding order of rings does not matter.
inner rings may extend outside their outer ring. The portion of inner rings outside their outer ring is dropped.
inner rings may touch or overlap each other.

Output

For non-empty results, output will always be a MultiPolygon containing one or more non-overlapping, non-edge-sharing Polygons. The GeoJSON spec is followed, with the following notes/modifications:

outer rings will be wound counter-clockwise, and inner rings clockwise.
inner rings will not extend outside their outer ring.
rings will not overlap, nor share an edge with each other.
rings will be self-closing.
rings will not contain repeated points.
rings will not contain superfluous points (intermediate points along a straight line).
rings will not be self-touching nor self-crossing.
rings may touch each other, but may not cross each other.

In the event that the result of the operation is the empty set, output will be a MultiPolygon with no Polygons: [].

Correctness

Run: npm test

The tests are broken up into unit tests and end-to-end tests. The end-to-end tests are organized as GeoJSON files, to make them easy to visualize thanks to GitHub's helpful rendering of GeoJSON files. Browse those tests here.

Performance

The Martinez-Rueda-Feito polygon clipping algorithm is used to compute the result in O((n+k)*log(n)) time, where n is the total number of edges in all polygons involved and k is the number of intersections between edges.

Settings

Global settings are set via environment variables.

POLYGON_CLIPPING_MAX_QUEUE_SIZE and POLYGON_CLIPPING_MAX_SWEEPLINE_SEGMENTS: Aims to prevent infinite loops - usually caused by floating-point math round-off errors. Defaults are 1,000,000.