# Long complex 3D curve

I need to model an object that has a complex 3D curve that defines an edge of a surface.
If use the curve through points tool, I just cannot get it. Some parts of the curve are short tight radius, others are long sweeping arcs that need to be straightish along one length and then smoothly transition into gently curve in the dimensions at the end.

I understand how 3rd order splines factor in previous points, so end up fighting with the underlying math that governs theses curves by either trying to move all the points such that i get what I want (a virtual impossibility) or putting down a ton of points to create a change between long sweeping curves and tight ones (doesnt look good and is impossible to edit smoothly)

Perhaps the answer is to build a line for each different section and butt them together such that they soothly transition into each other?

How smooth are the transitions? Degree 3 splines have continuous curvature (G2) while many manufactured objects have tangent continuity (G1) between sections of edges and surfaces. One way to accommodate G1 continuity is to build the curve for each section, and then match the curves using Match with the desired level of continuity. Another way is to stack control points at the G1 transitions but this can lead to problems if the curves are used to generate surfaces.

I have watched several training videos on this butit is still not clear to me how to do this.
The transitions must all be seamless. If i use multiple curves and then match or join them (ot sure when I should use which) then wherethey join need to be tangential or normal to each other.

these curves are being used to create surfaces. Using multiplte points at each dramatic transition kind of defeats the purpose of using NURBS to my thinking, plus its really clunky and the shape need to be noodled a lot before itis finalized so all those points have to be dragged around and possibly individually tweaked with every change.

I watched a vide for another product where tow designers used two different techniques to model the bosy of a human powered aerodynamic verhicle. They used T-Splines. I was left wondering if Rhino supots these and what they are all about. It looked like they were creating a poly surface with them rather than a NURBS curve.

I have read the Rhino Manual but aside from a thorough discussion of the controls, there isno real tutorial on complex body shapes with lines that mustbe followed rather than defined bythe NURBS math.

Proterio,

Join will leave kink points at the junctions if the curves are not already tangent or have continuous curvature (G2), whereas Match will allow you to specify the continuity and adjust the ends of the curves to achieve it.

If you build your curve from multiple sub-curves, you might consider using blend or adjustable blend curves between segments, this will make smooth transitions. Once you join them, consider using Rebuild (easy way to get rid of kinks and make the structure of the curve balanced), and try rebuilding to 5 degree instead of 3 degree, 5 degree tends to make a smoother curve and thus a smoother surface. If there are too many points, use Fit Curve to keep the points dense where you need them and not where you donâ€™t. If you need to tweak the curves, try Soft Move instead of adjusting many points, and if the points get funky (the curve bends stiffly at a point, especially after you have inserted or deleted points), try Make Uniform.

Yes Rhino supports T-splines as a plug in, very handy for types of work where you need flowing transitions between volumes that go beyond filleting. It is a different approach to modeling, but it dovetails nicely with Rhino NURBS.