Two-manifold vs. non-manifold polygonal geometry

User Guide > Modeling > Polygonal Modeling > Polygons overview >

Polygonal geometry can have different configurations or topology types in Maya. Understanding the characteristics of these topologies can be helpful when you need to understand why a modeling operation failed to execute as expected.

Two-manifold topology polygons have a configuration such that the polygon mesh can be split along its various edges and subsequently unfolded so that the mesh lays flat without overlapping pieces.

Non-manifold topology polygons have a configuration that cannot be unfolded into a continuous flat piece. Some tools and actions in Maya cannot work properly with non-manifold geometry. For example, the Boolean operations and the Reduce feature do not work with non-manifold polygon topology. The image below shows three examples of non-manifold topology polygons.

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.
Delete Face.
Edit Mesh > Merge 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.