Rhino Modeling and Higher Curve Degree

When do you use curvature degrees of more than 3, and how often? For what type of products?

Talking about Rhino modeling primarily, however this applies to all Nurbs Modeling…

Not sure about Rhino but I prefer Alias for building.

Curvature continous is when you have 3 cvs aligned to the gradient of a point on a curve or surface. Tangent continous have 2. The difference is, in the case of tangent, the highlight will stay at a region, whereas continous curvature gives you a nice controlled transition of highlight.

My instructor said, according to GM’s standard, everything should be curvature continous unless the radius is below 0.3mm( or maybe it was 3mm). That’s for cars and interiors. It depends on what you want to achieve. If you meant to have a controlled highlight over a surface transition, then use curvature continous. If you want a more static look, use tangent. It’s not the matter of which is better, it the matter of the design intent.

Just build a simple corner with both types of transitions and observe how the highlight rolls as you move the model. Then ask yourself which is what you are looking for.

Thanks MC

Was wondering about higher degrees like 7-9, Rhino has these, if not all the way to 11… When would you use this level?

Rhino has curvature continuity, and if you can accomplish the range between car models and say, creature or character modeling with just degree 6, when to use the other levels?

They are there for something, just haven’t found what that is.

The highest I go is 5th degree curves. I think other than surface properties which I am not sure about, it’s sometimes due to personal preference. I’ve not really needed to use any thing above 5th degree. If the surface is complicated, I prefer to break it down into smaller sections.

Another instructor likes to use 2 span edit point curves to get natural looking acceleration. I usually lay a 3rd degree curve, then shift the cvs, then bump it up to 5th degree if it’s not enough.

There’s a lot more to learn, and it takes both experience and a good teacher to understand everything. In Rhino, you can just push buttons and hope it works for you. In Alias, you got to learn the theory first then apply.

The goal, as I understand it, is to use the lowest degree curve necessary to create the line you want in the fewest possible spans. Try drawing a degree 3 curve first, but if there isn’t enough flexibility, switch to higher order curves one step at a time until you get the control you need.

I try to avoid multi-span curves in production (or late-in-the-process) surfacing to keep the data light and because I’ve always been taught that single-span curves are preferable to multi-span curves (someone else could better tell you why). Most people probably don’t care and build their data however they want to, but if the goal is to create surfaces with no (or few) isoparms, internal edit points are a no-no. Plus, internal edit points can create G3 continuity breaks in degree 3 curves. This is undesirable if you are creating a surface that will be polished or reflective.

I could ramble on for a while, but to answer your question directly,

I increase curve degree when:

  1. I want to create a shape a lower order curve can’t create without adding multiple spans.
  2. I need curvature (G2) or higher continuity at both ends of a curve.

I change curve degrees quite frequently, but always try to keep the degree as low as possible for what I need. Degree 1 for straight lines. Degree 3 for simple shapes and tangency at endpoints, higher degrees for more complex shapes or greater continuity levels (G2 on up) at endpoints. Using unnecessarily high curve degrees means you have to manage a lot of cv’s, which takes time and is a nuisance.

This applies to almost any type of product where you want a maximum quality surface, but it becomes increasingly important as surfaces grow in size and/or increase in shinyness or reflectivity. Automotive surfacers could probably write encyclopedias on what they know about high-order geometry.

Much of this is what I understand to be universally good surfacing strategy, but if you’re just making models for fast renderings or to quickly capture a concept, things like multi-span curves and surface isoparms are much less of an issue.

Oh, and I’m an Alias user, so hopefully my terms translate to Rhino.

Pro/E is limited to degree 3 geometry, which I think is lame, but that’s not what this thread is really about.

I hope this helps. Perhaps if there are any transportation surfacers (ufo?) on the forums, they can add more. Hmmm, come to think of it, you should post this question on the forums at cardesignnews.com and see what they say.


Thanks for the reply.

BTW, would you want to use Pro E for something like a toy car. I am talking a realistic toy car, something comparable to a 1/18th scale Diecast, only requiring plastic injection molded parts.

Obviously, I am confortable using Rhino, but would I require Pro E to create prototype parts?

I am building a “final” surfaced model in Rhino, obviously, however I’ve heard you can go directly to manufacturing level without using a Solids export.

You can machine from surface data, but most people prefer to work in solids at some point, because it is easier to create ribs, bosses, and other internal features in that type of program. In a surface program, you’ll have to build such features up surface by surface, whereas programs like Pro/E have tools for creating them all at once (a rib tool, a solid extrude tool, a hole tool, etc).

If your Rhino surfaces are built well and stitch properly, you can use the surface data in Pro/E, but engineers typically resist this type of thing, citing many reasons, most of which revolve around the fact that rhino/alias data isn’t parametric when it’s imported. This makes the model more difficult to update if something has to change.

Actually, the difference between “solid” and “surface” modeling is semantics, it’s a matter of interface design and target market. They’re all essentially “surface” modelers since that’s what the geometry is in the end.

Just as an example, Rhino V4 will have specific tools for making “holes” and “ribs,” (hopefully)improved shelling, etc. “Solid” modelers traditionally don’t have freeform surface editing not because of anything about “solids” preventing it but because it’s phenomentally cumbersome (looking at VX, the only product I’ve seen that does this) to integrate point-pushing into the “history tree” of a fully parametric model, and their primary market was engineers making assemblies of relatively simple machined parts.

Regards to the original posting. Using degrees higher than 5 is just a matter of preference. Techically such curves/surfaces are “smoother,” like the difference between degree 2 and 3 or 3 and 5, but the difference is at that point academic for any real-world purpose–3 being sufficient for freeform shapes in most cases. The only visible difference is that moving a control point around has a more subtle and ‘spread out’ effect on the surface.

Rhino WIP4 (work in progress 4) has Bass ‘feature’ and rib ‘feature’.
just thought I’d let people know.

And one more point on topic.
It is possible to get a lighter file size with higher degree than with lower degree and more objects. This is not necessarily true as there could be finctions taking up way more characters, hence bits. But apparently as per other people’s experience it is generally true. Although I do not know anyone that would intentionaly up the degrees to get a lighter file.

Look at this:) 404
I built it in Rhino