Considerations for Modeling Component Surfaces

There are several considerations when modeling the surfaces that will be assembled into a surface mesh:

All surfaces must be either cubic NURBS in both the U and V directions, or linear NURBS in both U and V.

The SCM (Surface Continuity Manager) does not work with quadratic NURBS.
Surface meshes work by locking boundary points together then managing the continuity across the junction. Each boundary point can match a single point on another surface's boundary. You can insert and remove knots to create the necessary boundary points. For more information, see Adding Knot Curves and Removing Knot Curves.
All junction boundaries must be open.
SCM uses the next row of points after the boundary to calculate continuity. Make sure that there are enough rows between two junctions: preferably two or more.
The assemble operation uses distance to determine whether two points across a junction should be locked together, so make sure points are overlapping or very close together. You can apply the Snap Boundary operator to help line up points along boundaries.

Junction Types

Surfaces can meet at an edge (I Junction), a T junction, or a star junction. Alternatively, an edge of one or more surfaces may be collapsed to a single point as at the poles of a sphere.

I Junctions

In an I junction, two surfaces join along a common border. This is the simplest of all possible junctions.

T Junctions

A T junction joins three surfaces. The border of one surface is joined to the borders of the other two. In addition, the other two surfaces share a border.

Star Junctions

A star junction brings several surfaces together at a point. You can have three surfaces (Y junction), 4 surfaces (X junction), or more. When using star junctions:

Make sure that the points around the star junction are more or less equidistant.


Good: Points around the junction are equidistant.	Bad: This junction will not deform well.

Make sure that there are at least four control points between two star junctions.
Avoid placing star junctions in areas of high deformation.

Collapsed Junctions

In a collapsed junction, one boundary of a surface collapses to a point (like at the poles of a sphere) and two opposite boundaries joins in a cone shape.

One boundary is collapsed to a single point (A), and a pair of opposite boundaries line up (B).

Multiknots

You can use multiknots to create discontinuities that help to line up surfaces. This is especially useful to eliminate holes where three or more surfaces meet in a complex way.

	Three surfaces meeting at a point can pose a problem.
	Joining them at the central point causes overlapping edges.
	To solve the problem, create knot curves of multiplicity 3 (shown here with an outline).
	The multiknots create a discontinuity that helps align the surfaces. Continuity is maintained across the multiknot curves as if they were junctions.

Local Materials and Textures

Individual subsurfaces can have local materials and textures. However, UV texture projections are not supported.

If you have already applied materials and textures to the individual surfaces, you have the option of copying them locally when you assemble. Be aware however, that the texture projections remain attached to the original surfaces — thus if you move the assembled surface mesh without moving the projection, the texture will slide.

To avoid sliding while moving the assembled surface mesh

Do one of the following:
- Freeze the assembled surface mesh. This also freezes the texture projections on it. However, you will need to manually reapply the surface continuity manager as described in Applying SCM Manually.
  
  or
- Constrain or parent the original texture projections to the assembled surface mesh.

Except where otherwise noted, this work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License