turf_pip

A small library for detecting if a point lies inside a polygon.

Features

• Works on polygons with holes
• Works with degenerate/self-intersecting polyons
• Returns PointInPolygonResult.isOnEdge if on the edge, PointInPolygonResult.isOutside if inside and PointInPolygonResult.isOutside if outside
• Not effected by floating point errors

Usage

Install via dart pub add turf_pip

import 'package:turf_pip/turf_pip.dart';

const polygon = Polygon(coordinates:[
  [
    Position.of([1, 1]),
    Position.of([1, 2]),
    Position.of([2, 2]),
    Position.of([2, 1]),
    Position.of([1, 1])
  ]
]);

pointInPolygon(Point(coordinates: Position.of([ 1.5, 1.5 ]), polygon)
// => PointInPolygonResult.isInside

pointInPolygon(Point(coordinates: Position.of([ 4.9, 1.2 ]), polygon)
// => PointInPolygonResult.isOutside

pointInPolygon(Point(coordinates: Position.of([1, 2]), polygon)
// => PointInPolygonResult.isOnEdge

Note: The input polygon format aligns with the GeoJson specification for polygons. This means that the first and last coordinate in a polygon must be repeated, if not this library will throw an error.

var polygonWithHole = Polygon(coordinates:([
  [
    Position.of([0, 0]),
    Position.of([1, 0]),
    Position.of([1, 1]),
    Position.of([0, 1]),
    Position.of([0, 0]),
  ],
  [
    Position.of([0.1, 0.1]),
    Position.of([0.1, 0.9]),
    Position.of([0.9, 0.9]),
    Position.of([0.9, 0.1]),
    Position.of([0.1, 0.1]),
  ]
]);

The library does not support multi-polygons.

Comparisons

Some rough comparisons to similar libraries. While point-in-polygon is slightly faster in most cases it does not support polygons with holes or degenerate polygons.

// For a point in a much larger geometry (700+ vertices)
point-in-poly-hao x 474,180 ops/sec ±0.55% (93 runs sampled)
point-in-polygon x 489,649 ops/sec ±0.75% (91 runs sampled)
robust-point-in-polygon x 376,268 ops/sec ±0.79% (89 runs sampled)

// For a point in bounding box check
point-in-poly-hao x 29,365,704 ops/sec ±1.30% (90 runs sampled)
point-in-polygon x 42,339,450 ops/sec ±0.78% (95 runs sampled)
robust-point-in-polygon x 20,675,569 ops/sec ±0.65% (95 runs sampled)

Algorithm

This library is based on the paper Optimal Reliable Point-in-Polygon Test and Differential Coding Boolean Operations on Polygons

Other notes

• Works irrespective of winding order of polygon
• Does not appear to be effected by floating point errors compared to point-in-polygon or robust-point-in-polygon