1.3.1 • Published 8 years ago

2d-polygon-self-intersections v1.3.1

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8 years ago


find self-intersections in a 2d polygon

This library may not be fast, but it is robust. Robust in the fact that it will find all of the self-intersections in a polygon - minus of course shared endpoints.

You can expect a time complexity of O(n^2)

Why wouldn't we use Bentley–Ottmann? We may in the future, but that is going to take some time and having a functional mechanism for detecting self-intersections is far superior to a non-existant one. The api won't have to change for this to happen.


npm install 2d-polygon-self-intersections


var isects = require('2d-polygon-self-intersections');

var poly = [
  [0, 0],
  [10, 0],
  [0, 10],
  [10, 10]

var r = isects(poly);
// outputs: [ [ 5, 5 ] ]


isects(polygon, filterFn)

  • polygon - an array of 2 component arrays (i.e. a triangle [[0, 0], [10, 0], [10, 10]]) or an array of objects: [{x:0, y:0}, {x:10, y:0}, {x:10, y:10}]
  • filterFn - a filter function called whenever an intersection is found: filterFn(isect, start0, end0, start1, end1, unique)
  • isect - current intersection (e.g. [5, 5]) - mutations in this array get collected
  • index0 - index of the segment (e.g 1)
  • start0 - start of the first segment (e.g [0, 5])
  • end0 - start of the first segment (e.g [10, 5])
  • index0 - index of the segment (e.g 3)
  • start1 - start of the first segment (e.g [5, 0])
  • end1 - start of the first segment (e.g [5, 10])
  • unique - boolean representing whether or not this intersection point has been seen before
  • return true to collect and false to discard

returns an empty array if no interesections or an array of 2 component arrays representing the intersection points.

NOTE: this library assumes the polygon is closed, so manually adding the start point as the end point has no effect.

Also note that there are 2 intersections per crossing, this library by default will only report one - all intersections will be unique. This behavior can be changed with the filterFn.