mesh function

Map<String?, dynamic> mesh(Map<String?, dynamic> topology, [ Map<String?, dynamic>? object, dynamic filter ])

Returns the GeoJSON MultiLineString geometry object representing the mesh for the specified object in the given topology.

This is useful for rendering strokes in complicated objects efficiently, as edges that are shared by multiple features are only stroked once. If object is not specified, a mesh of the entire topology is returned. The returned geometry is a shallow copy of the source object: they may share coordinates.

An optional filter function may be specified to prune arcs from the returned mesh using the topology. The filter function is called once for each candidate arc and takes two arguments, a and b, two geometry objects that share that arc. Each arc is only included in the resulting mesh if the filter function returns true. For typical map topologies the geometries a and b are adjacent polygons and the candidate arc is their boundary. If an arc is only used by a single geometry then a and b are identical. This property is useful for separating interior and exterior boundaries; an easy way to produce a mesh of interior boundaries is:

var interiors = mesh(topology, object, (a, b) => a != b);

See this county choropleth for example. Note: the a and b objects are TopoJSON objects (pulled directly from the topology), and not automatically converted to GeoJSON features as by feature.

Implementation

Map<String?, dynamic> mesh(Map<String?, dynamic> topology,
        [Map<String?, dynamic>? object, filter]) =>
    obj(topology, meshArcs(topology, object, filter))!;