Manifold

Intuitively, surfaces are manifold if you can draw a regular coordinate grid when zooming in far enough. For example, earth looks flat when near the ground.

Not all shapes are manifold.

Polygon Meshes

A manifold polygon mesh has fans, but not fins.

There are two conditions to check:

  1. Every edge is contained in only two polygons (no “fins”)
  2. The polygons containing each vertex make a single “fan”