# Hull.js examples

### Basic Usage

To get concave hull (polygon) by a set of points just call `hull` function:

``````var pointset = [[251, 275], [248, 276], [255, 272], [246,278], ...];
var pts = hull(pointset, 10);
// ... draw pts``````

You can adjust concavity of a hull via 2nd param:

``````var pointset = [[251, 275], [248, 276], [255, 272], [246,278], ...];
var pts = hull(pointset, 80);
// ... draw pts``````

### Performance

Hull.js also works well with large data sets. This example has more than 52K points, `concavity = 20`. Execution time in your browser: ms.

### Multiple Shapes

If you want to get hulls of a point set that looks like a multipolygon, then you need to call `hull` function for each subset (for each polygon). In this example we use LukaszKrawczyk's density based clustering library (but you can use any other implementation) to split one large point set to several groups, after that we get hulls for each one:

``````var clusters,
dbscan = new DBSCAN(),
pointset = [[14, 18], [22, 19], [23, 20], ...];

// get clusters
clusters = dbscan.run(pointset, 250, 50);
clusters = clusters.map(function(cluster) {
return cluster.map(function(i) {
return pointset[i]; // map index to point
});
});

// get hulls
clusters.forEach(function(pointset) {
var pts = hull(pointset, 30);
// ... draw pts
});``````

Otherwise (without clustering) you'll get something like this:

``````var pts = hull(pointset, 30);
// ... draw pts
``````

### Geo Points

If your point format differs from the [x, y] then you can provide a custom format pattern:

``````var pointset = [
{ lng: -0.2067, lat: 51.4911 },
{ lng: -0.2070, lat: 51.4915 },
{ lng: -0.2074, lat: 51.4912 },
...
];

var pts = hull(pointset, 0.0011, ['.lng', '.lat']);
// ... show pts on map``````

### Geo Points on a Globe

All calculations in hull.js are based on cartesian coordinate system. If you use it for calculation and data visualization on the global map, don't forget that globe has the shape of geoid, latitude and longitude are angles (not points with X and Y), and after projection we have some map distortion. So, for the better accuracy you need to convert latitudes and longitudes to cartesian coordinates, calculate concave hull and then convert points back to the geo points. For example (using Leaflet library):

``````var pointset = [
{ lng:-20, lat:90 },
{ lng:-20, lat:80 },
{ lng:-20, lat:70 },
...
];

var pts = pointset.map(map.latLngToContainerPoint.bind(map));
pts = hull(pts, 58, ['.x', '.y']);
pts = pts.map(function(pt) {
return map.containerPointToLatLng(L.point(pt.x, pt.y));
});
// ... show pts on map``````