Whats wrong with multi span/knots?


#1

Hi,

I am trying to figure out why bezier curve is better than multi span curve or surface…From my understanding A class curve/surface is smoother but I did a little experiment.

I build a curve with 6cps and degree 5 and then used fitcrv which produced 11cps, degree 3 and 9 knots. I run crvdeviation and it came up with 0.000239…cm. So how is it smoother? It’s almost identical

Then I made a surface from both curves to check reflections/continuity and again it looks identical.


The only difference is in curvature graph. The breaks between knots are visible at the ends but why does it even matter if the surface appear to be the same when analyzing it with zebra and emap?

Finally I moved one of the points up by 0.5cm which resulted in even bigger break in curvature graph of 8 span surface while A class srf graph remained smooth

Now the changes are visible in reflection of surface and they don’t appear to be as smooth anymore.

So to conclude, correct me if I am wrong but A class surface is easier for editing while maintaining the smoothness but if we create multi span surface and don’t move its control points it looks equally smooth so why is it bad idea to be using them? Why is it not prefered option for A class modeling?


(Rob McPherson) #2

Two points here-

  1. When using a multispan you probably will be using it to achieve greater variations than in your example. Why would you use 11 control points on a very gentle curving surface??! So therefore those curvature graph spikes will absolutely show. The goal is to model with as few control points as possible.

  2. The single span will always stay smooth and fair. Always. And be easier to work with.

If you’re not dealing with styling surfaces that are ‘customer facing’ then don’t worry yourself about single spans.

Regards,

Rob.


#3

If you are manufacturing this part it is unlikely to make any difference. The only possible difference would have to do with how well your part translates to other software.


#4

Hi Radovan - I think you’ve answered your own question. Single span surfaces ease the editing process. In your example, if you were to split the surface made from the rebuilt/degree 3 curve at the knots, you would then have class A surfaces - just smaller ones of a lower degree than the original. Managing the joins between these surfaces can become a headache if you need to ensure (curvature) continuity. Equally, you could also imagine your degree 5 surface being part of a larger set of surfaces that may also require management of continuity. It’s all a question of context.


#5

Thank you for reply… I understand that the goal is to model with least amount of cvs etc…but the question was why?..why is it the goal if sometimes you can achieve almost the same result using more complex geometry?..I just think that in some cases it is easier to add a knot to a surface than worry about not using it at all and thinking how to lay out surfaces because you are not allowed to use more than 1 span…maybe its just me but I find it easier in some cases


(Rob McPherson) #6

Oh, you’re absolutely correct there. It’s definitely easier to lay down multispans to start with but managing the curvature matching over different patches will be harder (as MattE has said) and you’ll end up with those non-smooth graphs somewhere.

I guess ClassA modelling is just being super-fussy. Again, only use it where you need to.

I have heard people say that if you use degree5 or 7 multispans then they should have enough internal smoothing to avoid the spikes. But I still think you loose a level of control where the knots are. Don’t know, haven’t really looked into this yet. Could be worth a try?

Regards,

Rob.


#7

You can use InsertKnot to add knots to a single-span surface in a way that leaves the actual surface completely unchanged, so the curvature would be identical either way.

I think the main difference is that single span makes it harder to get yourself into trouble, while multispan gives you more rope. That said, it’s possible to have great multispan surfaces, and it’s possible to have crappy single span surfaces. I think some people prefer to only use single-span, as a way to help enforce discipline.


(Rob McPherson) #8

But then you’d end up with a multispan wouldn’t you? I’m not sure just adding in knots into a single span surface and then leaving as-is would be particularly useful? I’m guessing most people would then go on to tweaking the multispan?? Or have I missed something?


(Pascal Golay) #9

Hi Rob -
No, I think the point was just that the number of spans is not in itself an indication of “quality”.

-Pascal


(Rob McPherson) #10

Ahhhh, I see now… thanks.
Apologies Tom.