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NURBS curves: 'degree' setting causes unpredictable results

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I'm pretty much completely baffled about what the 'degree' setting on NURBS curves actually does. Here's what teh VW help guide says:

 

Quote

The curve degree affects the number of vertices created; the higher the degree value, the greater the number of vertices.

 

Here's what happens when I try various things:

(when I write "created with" I mean the number I put in the "curve degree" box of the NURBS tool settings)

 

482005158_ScreenShot2019-10-14at18_20_38.thumb.jpg.6820601c600deb1fbdad9d1007bcd467.jpg

 

1047262960_ScreenShot2019-10-14at18_24_15.thumb.jpg.df14073f7e3b7b85eae857ea5ecd8617.jpg

 

Each of those shapes is created by clicking on the same 4 points in the same sequence.

 

Here are some questions for starters:

 

"Interpolation mode"

1) Contrary to what VW help says, changing the 'degree' in the NURBS tool settings doesn't affect the number of vertices. It does seem to result in a slightly different shape though. So what does it do?

2) If I change the 'degree' number in the OIP, what exactly determines the number of vertices I end up with?

3) If I change the 'degree' number in the OIP, why does a line with degree 3 end up with *more* vertices than a line with degree 4?

4) Is it intended behaviour that changing the 'degree' number in the OIP flips an "interpolation mode" NURBS into being a "control point mode" NURBS?

 

"Control point mode"

5) Why does the OIP tell me the NURBS has a different degree from the one I created it with?

6) Why does a created-as degree 3 or 4 NURBS have fewer vertices than a created-as degree 2 NURBS?

7) Why does on 'degree 4' NURBS have a different number of vertices compared to another 'degree 4' NURBS (and same applies to the two degree 5 NURBS)?

8 ) Why does one of the degree 5 NURBS have fewer vertices than one of the degree 4 NURBS?

 

Final question: is it user error, bugs in the software, or bad documentation that's causing the confusion here?

 

(I've attached the file below)

 

NURBS.vwx

 

 

 

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I agree about the confusion of NURBS degree changes, but have decided to just get over it.  It could have something to do the the underlying math, or with vwx interpretation of it, or something else entirely. Despite this condition I find that I can usually make NURBS do what I want.

 

Of greater concern to me is an apparent preference in vwx for the Control Vertices in NURBS rather than the Interpolation.

If degree is changed, I don't want my interpolation points to all change to control points.

If I convert a 2d poly cubic spline to NURBS, I want the NURBS to have Interp points, not Control, etc, etc.

A Conversion option is offered in the Mode bar, but it only allows toggle between corner (ruled) point and curved point.

I often wish I could also toggle between control and interp.

 

Basically, I usually want to have the curve pass through the points.

 

So, whine whine whine

 

-B

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Yes, what started me off on this was when I got confused about why a NURBS curve had seemingly changed to control points without me asking it to. I think it was because I was fiddling with the degree number.

 

Are there situations where changing the degree is useful?

 

Like you my solution for now is to 'get over it' simply by never touching the setting, seeing as I don't understand what it does. But if VW has a setting like this, it ought to be properly explained in the documentation.

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The VW help file is not very informative when it comes to curve degrees, here is a bit better explanation:

 

https://knowledge.autodesk.com/support/maya/learn-explore/caas/CloudHelp/cloudhelp/2015/ENU/Maya/files/NURBS-overview-Degree-of-NURBS-curves-and-surfaces-htm.html

 

and

 

https://www.rhino3d.com/nurbs

 

 

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2 hours ago, JMR said:

The VW help file is not very informative when it comes to curve degrees, here is a bit better explanation:

 

https://knowledge.autodesk.com/support/maya/learn-explore/caas/CloudHelp/cloudhelp/2015/ENU/Maya/files/NURBS-overview-Degree-of-NURBS-curves-and-surfaces-htm.html

 

and

 

https://www.rhino3d.com/nurbs

 

 

Thanks - I've maybe 60% understood the explanation in the Rhino link.

 

I don't think it explains what Vectorworks is doing though. For example I think I've understood that a higher degree is not necessarily related to a higher number of control points, which is what the VW Help explanation and/or edit behaviour implies.

 

I've also been looking at this GIF, where the degree setting is changed without affecting the control points, and where it can switch between the equivalent of 'interpolation' and 'control point' modes as @Benson Shaw was describing above (and is not possible in VW)

 

https://en.wikipedia.org/wiki/File:Spline01.gif

 

Spline01.gif

Edited by line-weight

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@line-weight I find changing degree applies well to NURBS surfaces and the related Interpolated NURBS surfaces.  I often find need to manipulate center of a NURBS surface, eg along the normal, esp those which start as conversions from planar objects. The surface has handles only at perimeter or bounding box corners. Raising U or V degree or both can provide the extra point(s) required near center of surface.

 

I usually use the Reshape>Add mode to place additional points, rather than the degree setting.

 

-B

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