Subdivision takes a low-polygon control cage and produces a smooth surface by recursively splitting and averaging. It is how most film-quality hard surface and character work is built.
The cage and the result
You model the cage — a few hundred polygons — and the algorithm produces the smooth surface. Editing stays fast because you edit the cage; the result stays smooth because the algorithm handles it.
What it needs
- Quads. Subdivision algorithms are defined for quads. Triangles and n-gons subdivide, but produce poles and uneven curvature at the seams.
- Even topology. Uneven cage density produces uneven smoothness.
- Supporting loops near any edge that should stay sharp. Without them everything rounds off equally.
Creasing
Most implementations allow marking edges as creased, holding them sharp without extra geometry. Convenient and less portable — creases are stored differently across formats and applications, and often lost in transfer.
Poles under subdivision
A five-edge or three-edge vertex creates a slight surface irregularity that subdivision cannot remove. On a flat area it is invisible; on a curved reflective surface it shows as a dimple. This is the practical reason to care about pole placement.
In real-time work
Subdivision is generally not used at runtime — the geometry is subdivided at authoring time and baked down, or the low-poly cage ships with a normal map. Some engines support runtime tessellation, but it is a specialised tool rather than a default.
With generated meshes
A reconstructed mesh is not a subdivision cage. It is dense, triangulated and irregular — subdividing it multiplies the irregularity. If you want subdivision behaviour, retopologise to a quad cage first; that is what the cage is.