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.
 |
 |
| [Figures 3, 4]
A model with no rows of controlling polygons. |
[Figures 5, 6]
A single row of controlling polygons. |
|
 |
 |
| [Figures 7, 8]
Additional geometry added at the corner. |
[Figures 9,
10] A model with two rows of controlling polygons. |
|
- 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.
