# 2d-polygon-self-intersections

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.

## Usage no npm install needed!

``````<script type="module">
import dPolygonSelfIntersections from 'https://cdn.skypack.dev/2d-polygon-self-intersections';
</script>``````

# 2d-polygon-self-intersections

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 Bentleyâ€“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.

## install

`npm install 2d-polygon-self-intersections`

## use

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

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

var r = isects(poly);
console.log(r);
// outputs: [ [ 5, 5 ] ]
``````

### api

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`.