Good Geometry vs. Bad Geometry
Common Problems With Geometry
- In the first example (the “T” shape), more than two faces share an edge. This is known as multiply connected geometry.
- In the second example (the “bowtie” shape), two faces share a single vertex without also sharing an edge.
This shape is also possible where two three-dimensional shapes share a vertex (such as two cubes meeting at a single point).
- In the third example, a single shape has non-contiguous normals (without border edges). That is, the normal on each polygon face points in an opposite direction. This is a less obvious example of nonmanifold geometry.
The following operations in Maya can produce nonmanifold geometry:
- Select Faces or Edges and select Edit Mesh > Extrude.
- Normals > Reverse (without extracting geometry).
- Edit Mesh > Merge Components.
- Delete Face.
- Edit Mesh > Merge Components to Center
You can convert non-manifold topology polygons into two-manifold topology (including the less obvious case of adjacent faces with opposite normals) using Mesh > Cleanup.
Invalid polygon geometry
Some types of polygon geometry will not work in Maya. Invalid geometry includes vertices that are not associated with a polygon edge and polygon edges that are not part of a face (dangling edges). While Maya does not let you create these types of geometry, it may be possible to import these types from other software applications.