MultiPolygon

[paomedia]

(enhancement)

As the GeoJSON format is concerned, I figure it out that "type":"MultiPolygon" is not supported. I suppose this would take an other kind of algorithm (idk?). Anyway, thanks for sharing !

[jradasaurus]

I figured this out the hard way too... the process would run out of memory and I noticed a pattern that all the ones that failed had multiple polygons. So I added a condition to my program to check if (obj.geometry.coordinates.length === 1) { and in my case I can ignore those... best effort :)

[HarryStevens]

If the GeoJSON feature is of type "Polygon", you can just pass the coordinates to polylabel. If it's "MultiPolygon", you can loop through the coordinates array to find the polygon with the largest area, and then pass that to polylabel. I do it like this, using d3.geoArea to calculate the areas:

function findPolylabel(feature){
  let output = [];
  if (feature.geometry.type === "Polygon"){
    output = polylabel(feature.geometry.coordinates);
  }
  else {
    let maxArea = 0, maxPolygon = [];
    for (let i = 0, l = feature.geometry.coordinates.length; i < l; i++){
      const p = feature.geometry.coordinates[i];
      const area = d3.geoArea({type: "Polygon", coordinates: p})
      if (area > maxArea){
        maxPolygon = p;
        maxArea = area;
      }
    }
    output = polylabel(maxPolygon);
  }
  return output;
}

[hollasch]

In my implementation, I plan to loop through all polygons, finding the most-interior point of all, and choosing the point with the greatest interior room as the winner.

[xeruf]

That algorithm has to be out there somewhere though, as MultiPolygons are successfully handled by the default OSM Mapnik style labels.

[peterqliu]

That algorithm has to be out there somewhere though, as MultiPolygons are successfully handled by the default OSM Mapnik style labels.

I think this is less a matter of computation, than of taste or preference. i would propose supporting multipolygons with two options:

• biggest polygon: get the pole for each subpolygon, returning the one that has the largest distance from its respective polygon
• concave hull: explode the multipolygon into points, and generate a single concave hull from them, and calculate the pole from that