Rule 6: Use Trimming for Boundaries that are not 4-sided
NURBS surfaces are inherently four-sided, they have four distinct boundaries. Sometimes a design requires a surface patch with 3, 5, 6, 7 etc. boundaries, or a hole in a surface.
The correct solution here is to trim a four-sided surface to the required shape:
Applying the Rule
Try to keep the number of n-sided patches to a minimum, by trying to find 4-sided patches wherever possible. Here are examples where n-sided surfaces were needed for the car model:
Applying this rule often means using some lateral thinking as you need to imagine (or guess) what four-sided surface could be trimmed away to create the surface you want.
Breaking the Rule
The rule can be broken by creating 3 boundaries that touch, or by having two of the boundaries tangent to each other. Both methods are bad practice and won't give good results.
If you do need to quickly fill a 3-sided patch for a rough concept model, then the Surfaces → N-Sided tool will probably give better results.
Theory and Practice
Degenerate Points
The methods shown above create a 'degenerate point', where a whole row of CVs are collapsed onto a single position. This creates very bad highlights and can cause many problems downstream, such as Rounds and Fillets failing, and so is to be avoided at all times.
This technique is used to create a sphere from a four-sided surface. This will often be acceptable, but a common technique is to trim out the degenerate point and replace the area with a square surface.
