[Figures 12 & 13] This shows how the NURBS surfaces convert to polygons (left) to create the framework for the polygonal model. The model (right) is shown after some preliminary editing.

Convert to Polygons
The NURBS surfaces are converted to polygonal meshes. The settings used to convert the NURBS surfaces to polygonal meshes vary depending on the density of polygonal mesh that the model needs to be. In Figure 12, the correlation between the conversion of the polygonal mesh and the NURBS surfaces can easily be seen. The NURBS surfaces and the polygonal mesh are superimposed on top of each other.

Initially, the distribution of the polygons is too heavy in some places and not dense enough in other places. The polygonal model has to be adjusted by hand in order to get the detail distributed in the right places. The polygonal model shown in Figure 13 was converted directly from the NURBS surfaces. This model has to be edited before it can be used.

Optimization
When a NURBS mesh is converted to polygons, the problem of badly distributed geometry must be addressed before the model can be used. The first problem that is normally taken care of is the problem of too much detail in the wrong areas. Some places on the head maintain a generally smooth topology. Places like this are the top of the head, the back of the head and the cheeks.

[Figure 14] Places like the top of the head are ideal locations for polygon optimization.

During the creation of a NURBS patch model, some additional isoparms are added to certain areas just to get the parameterization of these areas to line up with other parts of the model. For example, the top of the head will often get additional geometry that has been generated in the nose area because the top of the head rests directly in line with the nose. This is one of the complications that must be dealt with when working with NURBS surfaces.

Now that the model has been converted to polygons, this is no longer an issue. Now the high-resolution geometry on the top of the head can be decreased. This can be done by eliminating edges, snapping vertices to a common point (creating polygons with zero surface area) or by deleting vertices. Decreasing the resolution achieves two goals:

1. This reduces the geometric complexity of the area, which helps reduce the possibility of creating wrinkles and unwanted flashing in that area.

2. By reducing the number of polygons, the area will appear smoother when subdivision surfaces are applied to those polygons. (Figure 14.)