// list the triangles that belong to the alpha shape
alphashape = {
const filter = alphashapefilter(delaunay, alpha);
return new Uint8Array(delaunay.triangles.length / 3).map((_, i) => filter(i));
}
// compute the boundary of that list
alphahull = boundary(delaunay, alphashape)
// based on https://bl.ocks.org/jasondavies/1554783
function boundary(delaunay, members) {
const counts = {},
edges = {},
result = [];
let r;
// Traverse the edges of all member triangles and discard any edges that appear twice.
members.forEach((member, d) => {
if (!member) return;
for (let i = 0; i < 3; i++) {
var e = [
delaunay.triangles[3 * d + i],
delaunay.triangles[3 * d + ((i + 1) % 3)]
].sort();
(edges[e[0]] = edges[e[0]] || []).push(e[1]);
(edges[e[1]] = edges[e[1]] || []).push(e[0]);
const k = e.join(":");
if (counts[k]) delete counts[k];
else counts[k] = 1;
}
});
while (1) {
let k = null;
// Pick an arbitrary starting point on a boundary.
for (k in counts) break;
if (k == null) break;
result.push((r = k.split(":").map(Number)));
delete counts[k];
let q = r[1];
while (q != r[0]) {
let p = q,
qs = edges[p],
n = qs.length;
for (let i = 0; i < n; i++) {
q = qs[i];
let edge = [p, q].sort().join(":");
if (counts[edge]) {
delete counts[edge];
r.push(q);
break;
}
}
}
}
return result;
}
// based on https://bl.ocks.org/gka/1552725
function alphashapefilter(delaunay, alpha) {
const asq = alpha ** 2;
function dist2(p, q) {
return (
(delaunay.points[2 * p] - delaunay.points[2 * q]) ** 2 +
(delaunay.points[2 * p + 1] - delaunay.points[2 * q + 1]) ** 2
);
}
return function(i) {
let t0 = delaunay.triangles[i * 3 + 0],
t1 = delaunay.triangles[i * 3 + 1],
t2 = delaunay.triangles[i * 3 + 2];
return dist2(t0, t1) < asq && dist2(t1, t2) < asq && dist2(t2, t0) < asq;
};
}
One platform to build and deploy the best data apps
Experiment and prototype by building visualizations in live JavaScript notebooks. Collaborate with your team and decide which concepts to build out.
Use Observable Framework to build data apps locally. Use data loaders to build in any language or library, including Python, SQL, and R.
Seamlessly deploy to Observable. Test before you ship, use automatic deploy-on-commit, and ensure your projects are always up-to-date.