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:
- Loss of UV information, especially across mirrored axes.
- 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.
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 hard edge. 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 3 through 10, different examples of polygonal smoothing are shown.
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| [Figures 3, 4] A model with no rows of controlling polygons. |
[Figures 5, 6] A single row of controlling polygons. |
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| [Figures 7, 8] Additional geometry added at the corner. |
[Figures 9, 10] A model with two rows of controlling polygons. |
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- Sharp corners will smooth out if there are no additional rows of polygons inserted (Figures 3 and 4). 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.
- In Figure 5, 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 6, 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.
- In Figure 7, additional polygons were added at the corner in the image below left. This allowed the smoothing operation to behave more predictably in Figure 8.
- In Figure 9, additional rows of polygons were added along the edges. Notice how the highlights on the edges are confined to the two rows. In order to control flashing, a large face on a polygonal model that transitions into a smaller face must be separated by two rows of polygons. Figure 10 shows how the additional rows give the smoothing operation more control.
