Cancel
Showing results for 
Search instead for 
Did you mean: 

spline pole structure

Valued Contributor
Valued Contributor
Sorry for anonther question related to surfacing.
 
I learnt that a good pole sturcuture of a spline is important for building quality surface. I'm still confused about to what extent I should prioritize pole structure?
 
Below is a simplified example of reproducing a hand sketch curve. The two splines are almost of the same shape.
 
  • Top: 7 degree + 1 segment + not so good pole structure
  • Bottom:  4 degree + 4 segment + good pole structure
 
Roughly speaking, which one should I choose for surfacing?
 
Thanks!
 
Image.png
4 REPLIES

Re: spline pole structure

Gears Honored Contributor Gears Honored Contributor
Gears Honored Contributor

Higher degree splines are inherently "stiffer"; i.e. the poles need to be moved farther to affect a similar change in a lower degree spline. In the splines that you picture, I'd be hard pressed to say that one pole structure was 'better' than the other.

 

If a bunch of poles are tightly grouped in one section of the spline or the control polygon starts crossing itself, then I would reevaluate the pole structure.

Re: spline pole structure

Valued Contributor
Valued Contributor

Hi @cowski1   Thanks for your comments!

Re: spline pole structure

Phenom
Phenom
@surfactant,

In addition to @cowski's comments....

You have to define the scope of the work before you just go in and start creating 7 or 9 degree splines for the sake of matching. Is this Class A work and will this surface be an A surface or is it a B or C (not readily visible - think under the outside skin of a car body - not visible once installed) surface? The only reason you should be considering 7 degree splines is if you're working on an A surface and it's going to be a minimum of G3 continuity - if not, then either refine your spline and reduce the poles down to 5 degrees or break this spline up into smaller pieces where it changes curvature/direction and reduce the pole count down to 5 degrees.

Remember, having more than a single patch is acceptable if that's the best that can be done. In this case, I think you can do better, particularly since this spline changes curvature/direction twice.

Also, just because a previous designer created a spline as a long, single curve with multiple changes in curvature/direction doesn't mean you have to go down that rabbit hole as well. You will usually be better off creating smaller surfaces that have fewer changes in curvature then coming back and creating the secondary and tertiary surfaces.
-Tim

Re: spline pole structure

Valued Contributor
Valued Contributor

Hi @TimF  Thanks for your comments! Your guidance always greatly improves my understanding of the key issue.