README.md

A small library for detecting in a point lies inside a polygon

Features

• Works on polygons with holes
• Works with degenerate/self-intersecting polyons
• Returns 0 if on the edge
• Not effected by floating point errors

Usage

Install via npm install point-in-polygon-hao

import inside from 'point-in-polygon-hao'

const polygon = [
  [
    [1, 1],
    [1, 2],
    [2, 2],
    [2, 1],
    [1, 1]
  ]
];

inside([ 1.5, 1.5 ], polygon)
// => true

inside([ 4.9, 1.2 ], polygon)
// => false

inside([1, 2], polygon)
// => 0 to indicate on edge

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.

const polygonWithHole = [
  [
    [0, 0], [1, 0], [1, 1], [0, 1], [0, 0]
  ],
  [
    [0.1, 0.1], [0.1, 0.9], [0.9, 0.9], [0.9, 0.1], [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 381,184 ops/sec ±0.80% (87 runs sampled)
point-in-polygon x 285,734 ops/sec ±1.33% (91 runs sampled)
robust-point-in-polygon x 267,738 ops/sec ±0.78% (93 runs sampled)

// For a point in bounding box check
point-in-poly-hao x 25,822,227 ops/sec ±2.87% (86 runs sampled)
point-in-polygon x 30,321,920 ops/sec ±2.14% (91 runs sampled)
robust-point-in-polygon x 26,708,560 ops/sec ±1.13% (91 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
  • Added robust-predicates to deal with some floating point errors