Patch Surface Alignment Tools
Certain tools in Maya and other modeling packages as well, are specifically designed to assist in creating and maintaining tangency between surfaces after the surfaces have been built. These tools are used after the initial surfaces have been setup and separated into smaller patches.

Align Surface
The purpose of the align surface tool is to modify two existing surfaces by creating positional, tangential or curvature continuity between them. Try to tweak the values in the option boxes before you finish the align operation. At first pass, the results given from the align surface tool can seem somewhat disappointing. But by adjusting the different values, you can get different results for the align operation.

Stitch Edge
Stitch edge aligns two adjacent surfaces and works much like align surface, but stitch edges allow the user to stitch partial surface edges to adjacent surfaces using a control along the stitched edge that adjusts the amount of the surface that aligns along the edge.

Global Stitch
Global stitch works very differently than align surface. The process is very simple. Pick all of the surfaces you want to have aligned, then use global stitch. The problem with global stitch is that the surfaces do not behave the way they should all of the time. Global stitch does not maintain tangency between surfaces during animation setup and animation. If the settings are working, this tool will do a great job of keeping the surfaces tangent throughout production. Getting the tool applied correctly, however, can be tricky. Following are some rules for using global stitch:

• Always save before using global stitch. Maya allows for undo operations, but saving is still a good idea. Initial results appear satisfactory, but sometimes problems are discovered after it is too late to undo the operation.
• Use global stitch with the Keep Originals button off.
• Use global stitch only after the majority of the surfaces have been aligned using other alignment tools. Global stitch is the glue that holds the model together after it is animated. It is not the best tool for getting the surfaces to become tangent.

[Figures 15–18] Attaching and detaching the four borders surrounding a surface.

Attach and Detach
When you have four adjacent surfaces that have been rebuilt to the proper subdivisions, you may have a condition where they cannot be easily aligned using the align or stitch tools. A simple solution is to use the attach tool to align the surface. The technique used here is to find a place where four surfaces come together, attach two adjacent surfaces (on the U side of the surface, for example), then detach at the isoparm where the surfaces attached, attach the surface on the other side (on the V side), then detach. Continue until you have attached and detached all four surfaces on all four borders. Figures 15 through.18 show the progression of attach and detach around a surface.

Attach-Sculpt Surface-Detach
Attach-sculpt surface-detach is a variation of attach and detach. Often when you attach a surface, a ridge occurs where the surfaces come together. One way to get rid of this ridge is to smooth the surfaces that are attached before detaching them. The easiest way to edit these surfaces is to use the sculpt surface tool. This tool has a NURBS smoothing option that averages out the values of the CVs in the current selection.

Individual surfaces can be selected for editing with the sculpt surface tool, or individual vertices can be edited as well, using the Mask Unselected Vertices option before editing. The area that needs smoothing can be edited using the sculpt surface interactive brush, or an overall value can be set in the Max Displacement option box. Alternatively, the area can be smoothed using the Flood button. When you finish smoothing the points, detach the surface to restore the even surface layout.

Pulling Points
Until now, the major reason for having a grid of surfaces that align has not been obvious. When you manipulate points on a surface to align with other points on adjacent surfaces, you can see why I use this approach.

The only points on a NURBS surface where you have direct control on the position of the surface is at the edges. Any other place on the surface will have CVs and hulls that pull away from the surface. You can easily change the shape of a surface in the middle of a surface by pulling points. But, unlike the vertices at the edges, pulling points in the middle of the surface provides less direct control of where exactly the surface will fall in 3D space.

An additional row of hulls are inside the boundaries of each NURBS surface. If you look at a NURBS surface, you can determine the number of hulls by counting the number of spans in the surface, and adding 2. Because I am creating a grid where edges of surfaces line up with the adjacent edges of surfaces, I can snap points from one surface exactly to points of the adjacent surface.

The alignment tools do not completely fix alignment on a five-cornered intersection. Attach-detach provides most of the alignment along the surface edges, but to get the last little bit, you just gotta pull points.