@turf/kinks and @turf/unkink-polygon

[polizz]

The kinks package detects self-intersection in the following shape: https://gist.github.com/polizz/53bfb5d8cd0e7cac09ae6543e1c89528. When I run the shape through unkink-polygon, I get the same shape back out. Not sure what is wrong here. The unkink-polygon function does work on other self-intersecting polygons. Let me know if I can provide more information.

export function removeDuplicatesAndFormatGeoJson(wkt) {
  try {
    let cleanGeom = clean(parse(wkt));
    const hasKinks = get(kinks(cleanGeom), 'features[0].geometry', false);

    if (hasKinks) {
      const unkinked = unkink(cleanGeom);
...

[rowanwins]

Hi @polizz

Thanks for that test case. Our unkinkPolygon is based on the simplepolygon lib so we'll raise the issue upstream and hopefully we can work out what's going on.

Cheers

[DenisCarriere]

@rowanwins I had to refactor simplepolygon and I internalized the library into Turf to get the build to pass. Maybe it was something in the refactoring?

https://github.com/Turfjs/turf/blob/master/packages/turf-unkink-polygon/lib/simplepolygon.js https://github.com/Turfjs/turf/blob/master/packages/turf-unkink-polygon/lib/geojson-polygon-self-intersections.js

Might be worth re-including simplepolygon into Turf to see if it changed anything.

[DenisCarriere]

👍 Just confirmed that this is a bug 🐛

I'll send a PR with the test case included, we can investigate this more on the PR.

[DenisCarriere]

@rowanwins Darn... 🤕 I tried replacing simplepolygon with the upstream npm version and it still didn't fix the issue... at least it's not the refactored code the issue.

[polizz]

I don't know if it helps, but I was researching simplepolygon's issue list. There was an old issue where it was stated that a very vertical line tripped up it's self-intersection detection. The test shape I provided has just such a line.

[DenisCarriere]

🤔 ... Don't know what could solve that, maybe you can submit your test case to simplepolygon to have them fix it upstream and we can use it in Turf.

[rowanwins]

Hi @polizz

So I've done some more digging into this.

simplepolygon uses it's own self-intersection library rather than @turf/kinks. The geojson-polygon-self-intersections library is a bit more advanced. This explains why there are some anomalies between your results. For example the geojson-polygon-self-intersections lib provides some options such as reportVertexOnEdge, which is what is happening with your poly.

One option for turf would be to adopt the geojson-polygon-self-intersections as our kinks module. Currently our kinks algorithm is a basic brute force algorithm with no options.

It probably makes sense in my mind for the @turf/kinks & @turf/unkinkPolygon modules to be aligned. Thoughts @DenisCarriere

[DenisCarriere]

👍 @rowanwins Agreed, having both modules align would be the best approach using geojson-polygon-self-intersections, unfortunately that's not a quick fix and would require some serious effort in making that modification.

[barnabas-avalara]

Hi @DenisCarriere. I had to re-implement kink and unkinkPolygon functions in my own project because of this issue. It's ES6 and a little brute-force but it does the job. Cheers!

https://gist.github.com/barnabas-avalara/2a9d9be5edd84fe2da8e769a9a9c509e