How to get polygons from connected line segments

Question

I have a list of connected line segments. How to get the polygons.

let lines = [
    [10, 10, 200, 10],
    [10, 10, 10, 200],
    [10, 200, 200, 10],
    [200, 10, 390, 10],
    [200, 10, 390, 200],
    [390, 10, 390, 200],
    [10, 200, 10, 390],
    [390, 200, 390, 390],
    [10, 390, 390, 390],
    [200, 10, 200, 200],
    [10, 200, 200, 200],
];

Sample code visualisation:

// poly 1 would be something like this.
poly1 = [
    [10, 10],
    [200, 10],
    [10, 200],
];

You could build a Doubly Connected Edge List (DCEL) from your line arrangement and extract the faces. Here's a Javascript [implementation](https://github.com/yubowenok/LineArrangement), with a link to a cool demo. — RaffleBuffle, Apr 13 '20 at 04:57

score 2 · Answer 1 · answered Apr 12 '20 at 07:33

I'd structure the solution like this:

Go through each element in lines and match one line with the other, e.g. element 1 and 2 would match in [10,10]
Go through each pair of lines retrieved from 1. and seek through lines for a third line that matches the pair of lines unmatched start/end points. E.g. element 1 and 2 would have unmatched start/end points in [200,10] and [10,200] respectively, this would match with element 3.

Codewise I think the best structure is:

One function matchCoordinates(coordinateA,coordinateB) that takes two coordinate pairs and returns boolean match.
One function matchLines(lineA,lineB) that takes two elements from lines and utilizes matchCoordinates to find matching lines
One function findMissingLine(lineA,desiredLine) that returns boolean if lineA is the same as desiredLine, here I would include the edgecase that e.g. element 3 in lines is both [10, 10, 200, 10] and [200, 10, 10, 10]

How to get polygons from connected line segments

1 Answers1