The one question.
DBSCAN never computes a center. It only ever asks, of one point at a time:
are there at least minPts points within ε of me? If yes, that
point is core and the cluster grows through it — every point inside its circle is
swallowed, and each of those gets asked the same question. If no, the point is parked as
noise, though a later cluster may still reach it and claim it as a border point.
Every point ends up as exactly one of three things.
| Kind | Test | Drawn as |
|---|---|---|
| Core | at least minPts points within ε, counting itself |
a filled dot — the cluster expands through it |
| Border | not crowded itself, but inside some core point’s circle | a hollow ring — it joins the cluster but never expands it |
| Noise | neither | a small gray cross — it belongs to nothing |
Things worth trying:
1. Two rings, ε = 30. Press Run and watch one cluster crawl the whole way
around a ring and stop. This is the case k-means cannot do at any k and failed.
2. Smiley, ε = 30. Three clusters and nobody supplied k: two eyes and a
mouth, with the long curve kept in one piece — the exact answer k-means cannot reach at any
k.
3. Elongated — two long thin clusters, the identical points used on the k-means and
GMM pages. What's the difference?