Two-manifold vs. non-manifold polygonal geometry

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.