Hi Brian,

> I was wondering how extruded parts could be twisted, as
> they don't show any more control points.

Yeah this doesn't work only by manipulating existing control points.


> So do you manipulate the underlying NURBS equations
> to obtain a twist?

Kind of - it's working at more of an analytic level but it's more of a surface fitting process - the deformer goes through a type of fitting process where it analyzes the deformation and fits a new surface to represent it, subdividing things down and adding more control points as needed until the new surface is within a close enough tolerance of the "ideal" procedural deformation.

One of the nice things about this process is that you don't have to worry about the control point structure of the input objects - even something made up of just simple 4 corner point planes will automatically get refined as needed to produce an accurate enough result.

There are quite a few different mechanisms in NURBS modeling that work in this kind of "iterative-refinement-until-accurate-enough" type of method - stuff like Sweeping, surface/surface intersections, curve projection, etc...

- Michael