It kinda seems like the splines DO twist. I may be interpreting this wrong, but I made a quick model with a spline on a curved surface and the curvature combs did not lay on the surface but instead seemed to twist in 3D space. Below are three screenshots, one was without adjusting any handles at all, then the second are two views of the same thing after I adjusted the handles. It may be hard to see but one set of handles is actually close to normal to the surface while the other is sort of halfway between normal and tangent. At the inflection points in the curve instead of seeing the curvature pass through zero as it switches sides it does get small but then flips over the line in a twisting motion without ever going to zero. I then tried sweeping a line along this curve though, and the direction of the curvature combs didn’t seem to affect the orientation of the sweep.
I’m not sure what to make of this, except that I think Design-Engine is right in that the user is perhaps allowed too much control. Why not constrain the handles to tangent to the surface?
