polygon-offset/example/leaflet_multipolygon.js

Polygon offsetting algorithm, aimed for use with leaflet

L.Polygon.prototype._projectLatlngs = function (latlngs, result, projectedBounds, isHole) {
  var flat = latlngs[0] instanceof L.LatLng,
      len = latlngs.length,
      i, ring, area;

  if (flat) {
    area = 0;
    ring = [];
    for (i = 0; i < len; i++) {
      ring[i] = this._map.latLngToLayerPoint(latlngs[i]);
      projectedBounds.extend(ring[i]);

      if (i) {
        area += ring[i - 1].x * ring[i].y;
        area -= ring[i].x * ring[i - 1].y;
      }
    }
    area += ring[len - 1].x * ring[0].y;
    area -= ring[0].x * ring[len - 1].y;

    if ((!isHole && area > 0) || (isHole && area < 0)) {
      ring.reverse();
    }

    result.push(ring);
  } else {
    for (i = 0; i < len; i++) {
      this._projectLatlngs(latlngs[i], result, projectedBounds, i !== 0);
    }
  }
};


L.Polygon.prototype._project = function() {
  L.Polyline.prototype._project.call(this);
  if ((this._latlngs.length > 1) &&
    !L.Polyline._flat(this._latlngs) &&
    !(this._latlngs[0][0] instanceof L.LatLng)) {
    if (this.options.fillRule !== 'nonzero') {
      this.setStyle({
        fillRule: 'nonzero'
      });
    }
  }
};