Creating a Patch Boundary Network

When building a surface, all currently active curves are used to form the boundaries between individual patches. The points of intersection between all the active curves are found, forming a network of intersections and segments. This network can be displayed by selecting DISPLAY \ PATCH \ NETWORK or .

Figure 3:102. Set of active curves chosen to form a patch boundary network.

Figure 3:103. Intersections and segments (drawn as thicker lines) that will form the boundaries of individual patches.

When the surface is built, each segment will generate a patch boundary and each intersection a patch corner. Therefore, a separate patch is formed within each loop of segments. The patch boundary network is the most important factor determining the quality of the surface, and even whether it can be built at all. The choice of boundary curves is discussed in more detail in Creating a New Network, however, the main points to follow are:

In order for the surface to be topologically feasible:

• The network must not have segments that cross over one another. Roughly speaking, this means that parts of curves that look like they should intersect with each other must in fact intersect each other (see Missing intersections between curves).

• All patches must have 3 or 4 sides. The exception is where all the segments in a loop lie in the same plane, in which case there can be more that 4 sides (see Wrong number of patch sides).

In order for the surface to have good geometric characteristics:

• The active boundary curves must be fair.

• The number of individual patches should be kept to a minimum.

• The shapes of the boundary curves should follow the flow of the surface. Try to keep opposite edges of 4-sided patches roughly parallel.

• Wherever possible avoid creating 3-sided patches.

The easiest way to check whether these conditions have been met is to try to build the surface (see Building the Surface). The default surface options settings are usually adequate for this purpose - simply select PATCH \ REBUILD SURFACE or .