Triangles, however, are not defined as NURBS surfaces. They are defined as subdivided triangles, in a similar way that smoothed polygons behave. The ability to tessellate adaptively is reduced.

Maya behaves differently than RenderMan. The geometry can still displace better than any smoothed polygon model, and the areas where quads transition into triangles are treated differently.

These points can be seen as advantages that subdivision surfaces in Maya have over smoothed polygonal models.

Other advantages include the following:

1. Interactive editing of low-resolution polygonal cage while previewing high-resolution geometry without using plug-ins.

2. Many tools that allow quick editing, mirroring and conversion of subdivision surfaces.

In short, Maya has developed many tools that make subdivision surfaces look attractive. But it must be noted that this entity type is notoriously unstable. Before using this entity type on a production, test it carefully and often. Results attributed to using subdivision surfaces include these:

1. Loss of UV information, especially across mirrored axes.

2. Maya has an invisible node called the shape node associated with every piece of geometry in the scene. Maya uses these shape nodes in the dependency graph for many important functions. Using subdivision surfaces can cause geometry shape nodes to simply disappear. Digging through the hypergraph can get the geometry back, but only after a heart attack or two.

Figures 19 & 20] A model with no rows of controlling polygons.
[Figures 21 & 22] A single row of control polygons.

Detail
Detailing in polygonal modeling has to be done in combination with a way to preview the results, which is why the smoothing discussion was introduced before the discussion on creating detail. If the resultant model is going to be smoothed using subdivision modeling techniques, then the results of this additional process should be checked whenever a significant amount of work is to be done. When the lips are detailed, check them, when the ear is detailed, check it and so forth.

Detailing usually requires the model to be split along the areas where the model has a topological change. For example, the edge of the lip is not exactly a hardedge. But if the edge of the lip is compared to the side of the cheek, it is significantly sharper.

Creating detail in regions like this requires the process of adding additional rows of polygons along these areas. To create the ridge at the edge of the lip, a row of polygons is created at the edge of the lip; when this single row is subdivided, it becomes two or more rows, adding more definition.

When applying additional rows to create detail, it is important to understand how these rows will affect the final model. Some simple rules can come in handy when these conditions arise. In the examples in Figures 19–26, different examples of polygonal smoothing are shown.

1. Sharp corners will smooth out if there are no additional rows of polygons inserted (Figures 19 and 20). Additional rows of polygons at the edges and corners help control the way the geometry is smoothed. These additional rows of polygons are used to create areas of detail in the final model.

2. In Figure 21, the shading artifact that blends through the single row all the way to the corner is called flashing. A single row of polygons will not stop flashing along the face of the square. In Figure 22, the corner where the rows come together was not controlled by adding an additional polygon, so the corner was smoothed unpredictably. A single row of polygons works better than no rows at all but will not provide adequate control for detailed areas.