The choice of surface stiffness is analogous to the selection of a different weight of spline when drawing a curve on the drawing board. When drawing a smooth shallow curve you would select a stiff spline, and when drawing a curve featuring a high rate of change of curvature you would select a flexible spline.

Flexible splines are useful for modelling knuckles and discontinuities, otherwise try and work with the stiffest spline possible. Often a good compromise is to use flexible splines transversely and stiffer splines longitudinally.

Two factors influence the stiffness of a surface:

Setting of stiffness through the Properties dialog.

A number of stiffness settings allow for the specification of stiffness in the longitudinal (row) or transverse (column) direction.

There is no absolute value for the recommended surface stiffness since it is very much dependent on the surface shape you wish to achieve. In general, a longitudinal stiffness of 5 and a transverse stiffness of 4 offers a good starting point from where you can go up or down dependent on the surface shape you wish to achieve.

The stiffer the surface, the easier it is to get a fair surface. At the same time, a stiff surface is more difficult to achieve high curvature shapes with.

Number of control points.

You need at least the same number of control points in the direction of the surface stiffness. For example: for a longitudinal stiffness of 6 you need at least 6 columns of control points and for a transverse stiffness of 5 you need at least 5 rows of control points.

To get an idea why this is necessary, imagine a 2 column surface (only a forward and an aft edge column without any control points columns in between): the surface stiffness can only be linear.

Maxsurf will make sure the stiffness of the surface cannot be greater then the number on control points in that direction; i.e. if you have an order 6 surface and you reduce the number of columns from 6 to 5, Maxsurf automatically reduces the surface stiffness from 6 to 5.

Tip:

Use as little control points as possible on a surface that is as stiff as possible whilst achieving the required shape. This will result in a fair surface model.

The example below shows a surface with the same net but with two different stiffness’s. One surface is set to flexible in both the longitudinal and transverse directions and the other is set to stiff.

Local Influence

When using few control points, a control point movement may influence the whole surface, but when using many control points the control point's area of influence will be more local.

This is illustrated below, showing the extent to which the surface deforms due to a control point movement using a rectangular 3*3 control point net.

The movement of the middle control point on the top edge would result in, the following deformation.

Note:

The influence of the control point movement may be seen across the full width of the parametric surface.

If the net is increased to 9*3, keeping the stiffness (as set in the Properties dialog) the same, the result of a similar control point movement would be as follows.

Through the addition of extra control points:

§   the surface has become more flexible

§   the influence of the control point movement is much more localized.

Note:

The surface stiffness actually relates to the number of continuous derivatives of the surface. There are two special cases:
1. If the spline is order 4, the curvature is continuous and the spline is the same as a piece-wise cubic spline.
2. If the order is one less than the number of control points, it is a Bezier spline.

 Watch a video that shows you the effects that a change to the surface stiffness has on the shape of a spline.

 Watch a video that shows how a change in stiffness affects the shape of the surface.