Maya Rendering Guides

Tips for Polygon Modeling

To avoid any problems with your model, keep in mind the following tips as you work.

Keep Polygon Faces As Quads (Four Sided) First of all, it's much easier to view your models with one less edge through each face. If your aim is to create an organic, smooth surface later by using the Smooth command or by converting the surface into a subdivision surface, it's a good idea to use quad faces as much as possible from the beginning. Quad faces smooth and convert to subdivision surfaces without problems. Figure 3-7 shows both quad and tri faces being converted to subdivision surfaces. You should be aware that a polygonal model with nonmanifold geometry won't convert to a subdivision surface model. This may also be of concern if you plan to render subdivision surfaces in Mental Ray (one of the rendering engines that ships with Maya).

FIGURE 3-7 Quad faces and tri faces smooth differently.

Quad faces

Tri faces

FIGURE 3-6 Smoothed geometry flattens when additional geometry is added to the original mesh (shown in wireframe).

FIGURE 3-7 Quad faces and tri faces smooth differently.

NOTE Models built for integration into real-time game engines must often be constructed using triangular faces. A quad-faced model can be automatically turned into a tri-faced model by choosing Mesh | Triangulate.

Also, some of the most useful polygonal modeling tools in Maya, such as the Split Edge Ring tool, require a series of adjacent quad faces to create a continuous split.

Avoid Nonmanifold Geometry Manifold geometry relates to the arrangement of the polygonal faces in a mesh. If a surface could be unwrapped and laid out flat, then it is described as manifold. There are several arrangements of polygons in a mesh that will result in nonmanifold cases. One type of nonmanifold arrangement would be when more than two faces share the same edge. Three faces, shown here, that share the same edge would form a T shape. This arrangement should be avoided. Another way to create nonmanifold geometry would be to have the normals of two adjacent faces pointing in opposite directions.

Nonmanifold geometry will cause certain modeling operations to fail. A conversion to subdivision surfaces (another type of geometry that we'll look at in Chapter 4) will fail if the geometry is nonmanifold. In some cases, Mental Ray will not be able to render your model if it contains nonmanifold geometry. If you find that your model contains nonmanifold geometry, you can use the Mesh | Cleanup options to try and fix it. But, most likely, you will need to inspect your model closely and fix it manually by reversing normals or deleting faces. This is why it is best to always be aware of the arrangement of your polygons as you move along the modeling process.

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