Two-manifold and 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 mesh that can be split along its various edges and 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 Reduce feature, and Sculpting Tools 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 non-manifold geometry.

The following operations in Maya can produce non-manifold geometry:

• Extruding faces or edges with Edit Mesh > Extrude.
• Reversing normals, without extracting geometry (Mesh Display > Reverse).
• Merging vertices, edges or faces into one vertex (Edit Mesh > Merge or Edit Mesh > Merge to Center ).
• Deleting one or more faces.

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.