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