Overview

1. Build a mesh of intersecting curves to describe a surface.
2. Choose CrvNet Tools > New Network from the toolbox and pick all the curves.
3. Alias interpolates between the curves to create surfaces automatically.
   • You can continue to reshape, add or delete the construction curves, and the curve network surface will update.
   • Add sculpt curves to reshape the whole curve network surface by mapping the shape of a free curve onto the surface.

Terminology

Refer to the diagram at left for illustrations of the following terms used with curve networks:

1. Curve: one of the curves in the network.
2. Intersection: where two curves in the network cross at the exact same point in space. In the interface this is also called a vertex.
3. Curve segment: a section of a curve defined by intersections (vertices) at both ends. Many of the curve network tools work on curve segments, including the continuity tools. In the interface, and sometimes in this chapter, this is called an edge.
4. Region: closed area of a curve network bounded by either three or four curve segments (edges).
5. Curve network surfaces: the surfaces are distinct from the curve network itself. In fact, the surface can be pulled away from the network by sculpt curves.
6. Curve network: all the curves defining the surfaces. Alias will not build surfaces until all the curves in the network are valid.

Valid and invalid topology

Alias creates one surface for each valid region of the curve network.

All curve network surfaces are grouped under a single node, as shown in the Object lister or SBD window.

Under certain conditions, you may be able to create a single surface from the resulting curve network surfaces, by using the Surfaces > Combine Surfacestool.

The following types of closed regions are valid

• Four sided regions are fitted with standard NURBS surfaces.
  
• Three sided regions are fitted with three NURBS surfaces.
  
• Regions with T-junctions that can be “collapsed” into three-, four-, or five-sided regions are treated the same as regular three-, four-, or five-sided regions.
  

The following regions are not valid

• Regions with more than five sides.
  
• Regions with only two sides.
  
• Regions with T-junctions that cannot be “collapsed” into three, four, or five sided regions.
  

Other rules

• There can be only one curve segment between any two intersections.
  

General tips and notes

• Curve networks do have a performance penalty. Do not use curve networks for surfaces that can be constructed just as easily with the normal surface creation tools.
• Use more curves in areas with detail, and fewer in areas with little variation.
• To design very complex shapes, create the network with a few defining curves, then add more curves and view the incremental effect on the network.

  This lets you monitor continuity and topology requirements and identify problem curves quickly, which is more difficult when you add all the curves at once.

• Try to construct the network from curves that all have the same degree and the same kind of parameterization.
• Curve networks produce best results when every surface region is four-sided. This means a maximum of four surface regions meeting at any intersection.
• If you must have triangular or five-sided regions, keep them as simple as possible. Use simple curves with little variation.
• Avoid T-intersections by adding more curves to the network, or extending existing curves.
• The history viewer can be used to return to a network for further modifications.