Turf polygonize

[NickCis]

New Module @turf/polygonize

Polygonizes a set of Geometrys which contain linework that represents the edges of a planar graph. It's basically an implementation of GEOS's Polygonizer.

I ended up porting this to javascript because I wanted the same functionality as Shapely's polygonize function. This python framework uses GEOS'S C api as backend.

JSDoc

/** Implementation of GEOSPolygonize function (`geos::operation::polygonize::Polygonizer`).
 *
 * Polygonizes a set of lines that represents edges in a planar graph. Edges must be correctly
 * noded, i.e., they must only meet at their endpoints. LineStrings must only have two coordinate
 * points.
 *
 * The implementation correctly handles:
 *
 * - Dangles: edges which have one or both ends which are not incident on another edge endpoint.
 * - Cut Edges (bridges): edges that are connected at both ends but which do not form part
 *     of a polygon.
 *
 * @param {FeatureCollection<LineString>} geoJson - Lines in order to polygonize
 * @returns {FeatureCollection<Polygon>} - Polygons created
 */

Example

This example is the test found in test/in/complex.geojson

const polygonize = require('@turf/polygonize'),
    fs = require('fs'),
    input = JSON.parse(fs.readFileSync('./test/in/complex.geojson'));

console.log(JSON.stringify(polygonize(input)));

An example of input's LineString:

Turned into polygons:

[DenisCarriere]

@NickCis 👍 This is a great new module! I'll send over a review

[DenisCarriere]

👎 All source code should be written in ES5 (with the exceptions of tests could be written in ES6).

All TurfJS modules should be very minimal with only a single index.js & test.js file.

You should consider publishing polygonize as it's own module on npm and TurfJS could simply include it as a dependency.

• Convert code to ES5
• Only include a single index.js
• Only include single test.js
• Convert /src to external dependency (ex: polygonize)
• Add @private to other utils functions

[DenisCarriere]

• Convert to ES5
• Publish Polygonize as separate module
• Only include single test.js & index.js files as source code

[DenisCarriere]

Add @name polygonize

[NickCis]

Should be removed, you can use @turf/line-segment to easily solve this.

I've used @turf/meta :: coordReduce in order to iterate over the coordinates and segment the LineString.

I'm working on the MultiLineString support. Sorry for the delay.

[DenisCarriere]

@NickCis I've got MultiLineString support by adding @turf/line-segment at the top level. Converts all the Geometry into 2 vertex LineStrings.

module.exports = function (lines) {
    return polygonize(lineSegment(lines));
};

[NickCis]

I'm working some issues in the polygonize module, i'll update the yarn.lock file when finished

[DenisCarriere]

Awesome stuff! So far everything looks good to me.

[NickCis]

Finished update of polygonize package :)

[DenisCarriere]

Going to keep the basic function syntax in case anyone wants to expand the TurfJS method they can easily do so.

module.exports = function (geojson) {
  return polygonize(geojson);
}

[DenisCarriere]

Noticed you're making Polygon fail, any reason why this should fail? I can already see this error being very annoying, example if you have a collection of LineStrings and you polygonize them you then get a FeatureCollection of Polygons, if you keep adding more LineStrings to that FeatureCollection and re-run polygonize then it would fail since the FeatureCollection will have mixed contents.

It seems like throwing an Error against Polygons would make this module less practical.

[NickCis]

@DenisCarriere I thought it would be more coherent to throw an error than just ignore another input that isn't a LineString ([Collection]Feature/Geometry<[Multi]LineString>). The exception is a good way to tell the user "Hey, are you sure that you want to send me all these things that i do not understand?".

But, as you are a maintainer of the library, i think that you have a better understanding of the target audience. If you explain me what will the expected behavior of the user, i'll happily make the necessary changes.

[DenisCarriere]

@NickCis As for Polygon Features, you can simply just return itself and not need to polygonize Polygons.

if (type === 'Polygon') return feature

The only thing that really need to be strict is the output, as long as the output is always the same type.

As for Inputs, the more geometry types the better, you could even attempt to properly support GeometryCollection (I don't know if that works with your library yet).

[DenisCarriere]

Would be nice to polygonize with mixed contents (polygons & lines).

Just shooting it out there, maybe it's not a valid request.

Red: LineString
Blue: Polygon

[DenisCarriere]

Come to think of it, polygons with holes might get problematic... 😕 Might be best to leave out Polygons from the supported input Geometry types.

@NickCis Keep throwing that Error 👍

[NickCis]

The library correctly supports GeometryCollections (there is also a test for this, i think that i've forgotten to add it in turf).

As regards (Multi)Polygons as input, is a valid request, in a way i'll have to suppose they are a especial case of a LineString which starts and ends in the same point, i could transform them using @turf/polygon-to-linestring. I cannot store it and just returned, because, if there are is a line which creates a polygon inside the polygon, the starting polygon should not be returned.

And example is the following:

Red: MultiLineString
Blue: Polygon

The valid result is the 4 inner polygons, and not the one that contains them.

The problem with this approach is that the algorithm still assumes that lines are correctly nodded (i.e.: they only met at their endpoints). So, if the polygon doesn't have a vertex where the line starts, it won't work correctly. Perhaps, it could be useful to add a non default option to find all intersections and divide lines and polygons. I think that it will have a huge impact on performance. Does turf have a module that given polygons and LineStrings, it finds all intersections and add them as new vertex? (if not, well, i think that i'll be starting to write a new module, haha).

[NickCis]

@DenisCarriere I didn't read you answer before posting, sorry!

[DenisCarriere]

@NickCis This looks awesome, maybe that can be an entirely other module, I had attempted this before but it was getting too complex for the basic implementation I had.

I never finished it but this would be ideal for @turf/polygon-slice.

More info here on this issue #580

Here's what I started which was never finished. I like your graph approach.

[NickCis]

Ok!, I'll look the issue and the branch! We'll follow the topic there!. Thanks!

[maphew]

I'm not getting expected results from polygonize. Where is the appropriate place to ask questions about it? I'm a raw newbie to Turf and barely familiar with Node.

[DenisCarriere]

@maphew this is the right place to ask questions, or you can submit an issue with a sample code. Use this issue #702 as an example.