Region Functions

Converts H3 indexes to and from polygonal areas.

Fill (polygonToCells)

Fills a Geometry (Polygon or LinearRing) with the indexes who's centroids are contained within. Returns IEnumerable<H3Index>. Supports polygons with holes.

var reader = new WKTReader(geometryFactory);
var somePolygon = reader.Read("POLYGON ((-122.40898669969356 37.81331899988944, -122.47987669969707 37.81515719990604, -122.52471869969825 37.783587199903444, -122.51234369969448 37.70761319990403, -122.35447369969584 37.719806199904276, -122.38054369969613 37.78663019990699, -122.40898669969356 37.81331899988944))");
var filled = somePolygon.Fill(9).ToList();

Fill also accepts an optional second argument specifying the VertextTestMode to be used while checking containment during the fill operation:

• VertexTestMode.Center (the default) requires that the center vertex of a given index must be within the provided polygon

• VertexTestMode.Any specifies that any vertex of a given cell's boundary may be within the provided polygon

• VertexTestMode.All specifies that all vertices of a given cell's boundary must be within the provided polygon

LineStrings / TraceCoordinates

There's also a Fill method which takes a NTS LineString instead of Polygon, and is the equivalent of calling lineString.Coordinates.TraceCoordinates(). Tracing coordinatees works by drawing lines bewtween each vertex, estimating the number of cells between the vertices for the selected resolution, and then using NTS' LinearLocation.PointAlongSegmentByFraction to interpolate the indexes that make up that line. This is similar to the index.LineTo() method, except that instead of generating the line in grid space coordinates,TraceCoordinates interpolates lines utilizing the spherical coordinates (center) of each index.

GetCellBoundaries (cellsToMultiPolygon):

Returns a MultiPolygon containing all of the cell boundaries of the input set of indices.

var filled = somePolygon.Fill(7).ToList();
var multiPolygon = indexes.GetCellBoundaries();

This produces the second image shown above.