# Why does you have to match number of control points when making surfaces?

I’ve watched a bunch of tutorials such as this (https://www.youtube.com/watch?v=V8pyC6nbVRI)

and it seems like people want to make sure that if they are, say, lofting between two curves, then those two curves should have the same number of control points. Why is that? When I sweep with two curves and they are different in control points it works out just fine. Am I missing something?

It depends on what quality you need on the resulting surfaces. But it’s all about making a good foundation for the rest of the house (the final surfaces in this case). The control points will influence the “structure” of the final surface(s). You can also think of how framing a house can influence the quality of the surface of the final walls. A good job under the surface is a prerequisite for a good final surface.

If you try to follow the video you linked to you will find out that it is very (very) difficult to not end up with creases along the edges of the surfaces of the bonnet if you don’t follow the instructor’s advice.

It’s not a perfect metaphor, but if you have a curved bridge and build it with two beams from one direction and three beams from the other direction, it’s not going to make your life easier when connecting the beams in the middle. And if you then bend the finished bridge (which is what you kind of do when you have curved surfaces), the two bridge-halves won’t bend exactly the same, and so you would notice that something (the curvature) is changing in the joint at the middle of the bridge. Or, speaking about surfaces, you’ll end up having a big problem creating a surface without creases or even glitches between surface patches.

On a “simple” surface with no complex curvature you may get away with being less careful with the starting conditions, but high quality surfaces (like the bonnet) is very much about doing a good ground work, otherwise you are going to have big problems down the road. Like when you are building a house, make sure you create a good foundation. If the foundation deviates too much out of the tolerances, you will have to make endless adjustments when building the rest of the house.

// Rolf

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If you want more detailed explanations of how surfaces are influenced by curves and other preparation work, I recomment to read the Alias Golden Rules (10 rules) of which I link to the first rule, Read all of the rules and you’ll have a better understanding of why the instructor in the video clip was so careful about the curves and even distribution of Control Points:

Be aware of that the following documentation (several tutorials) is among the best on the topic of surface modelling that exist on the planet, and also that the same general rules apply for all NURBS modeling, so this is as relevant to Rhino users as it is for Alias users:

// Rolf

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Hello- one thing I would add is that it’s not just the number of points that needs to match, it’s the curve structure - See the attached file for some examples. The underlying assumption is that you generally want to make the simplest surface you can, with the most uniform structure, to avoid unwanted curvature changes and inflections, which become more and more likely when the point count goes up and the structure less uniform.

PointCountStuff.3dm (89.6 KB)

@jshethj - V5 now.

-Pascal

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Hi Pascal that file looks really helpful but it’s in version 6, can you save it down to 5 for me?

Thank you!

Thank you!