Smooth Nurb Objects

[Malte Koeditz]

I have a question about the smoothing of Nurb objects or rather why Vectorworks seems to be faceting Nurb objects automatically. I have attached a picture of an 'extrude along path' object - 'circle' extruded along a 'nurbs curve'.

Actually the same sort of facetting happens directly with a 'nurbs curve'.

Is there a setting I need to change for this or am just doing something wrong generally?

I would have expected that as a NURB is based on mathematics that it would work like other vectors and be infinitely accurate so maintaining a smooth curve in drawings.

Thanks for your help in advance,

Malte

[Miguel Barrera]

The circle is the object creating the facets and not the nurbs curve. You need to adjust the 3D conversion resolution in the VW preferences, which by default is set to medium.

[islandmon]

Although the polyline is faceted based on the 3dconversion resolution, as shown... conversion to NURBS will produce very useful surface renderings without the overhead of polygonal 3d mesh ( which is based on the polygon conversion resolution).

[RubenH]

There's a link about this kind of things.

http://www.hydraulicdesign.net/meshes.htm

Islandmon:

?Could you elaborate on "conversion to NURBS will produce very useful surface renderings without the overhead of polygonal 3d mesh"? please!

[islandmon]

Polygonal mesh is determined by the frequency of edge subdivisions. The higher the frequency the more tessellated the triangulation and the larger the file size.

NURBS on the other hand are infinitely tessellated because they are not edge dependent; instead they are controlled by Bezier control points which are weighted. Movement of the controls changes the curvature based on the algorithmic limits of the procedure. For purposes of surface editing more control vertices may be added and weighted to achieve the desired contours. NURBS can be lofted together to form complex shapes.

The difference between Mesh & NURBS is logically explained by two procedures

for determining the circumference of a circle:

1) Division of a polygon to achieve ever closer segmental approximations

or

2) Use the formula for circumference = 2piR

[Malte Koeditz]

Thanks for all your replies,

Both the 2D and 3D conversion resolutions are set to 'very high'. However the article on http://www.hydraulicdesign.net/meshes.htm (very useful) made me think that it is just a simplification by VW to minimize file size.

However this seems to be unecessarily over the top as it is also doing the same faceting for just a normal NURBS curve - both by control points and interpolation points. This makes me think that it is not the circle but the NURB itself?

Judging by your replies you don't experience the same thing? Am I being uneccesarily picky with the accuracy of the program/drawings? Is this affected by the scale setting of the design layer?

[RubenH]

The problem is that the values of 3d conversion resolution are preconfigured. Maybe the user should have, in an object basis and not the overall model, control over the resolution of 2d and 3d objects (regarding the resolution of the mesh, and the conversion resolution).

[Benson Shaw]

Malte - Just to confirm your findings: This is not an issue with your computer or VW preferences. Any VW NURBS curve displays as a faceted element. Zoom in to reveal a series of straight "segments". The NURBS curve is not composed of active vertices at each segment end. This condition obtains in all the 3d resolution modes, although the higher modes have shorter "segments".

-B

[Malte Koeditz]

So I think I understand the NURBS concept completely now.

However I am confused as to why Vectoworks preconfigures the 3D conversion resolution. Does that not to some extent defeat the purpose or rather benefit of NURBS modelling? As I understand the NURBS curve or line or whatever the object, is effectively specified with mathematical formulas allowing - ideally - for infinitely precise display. But VW is removing this clear benefit by having a preconfigured 3D conversion/display resolution.

[Benson Shaw]

Beats me why the conversion resolution affects NURBS objects in this manner. I think the programmers will tell us that it is computational overhead. Trading faster redraw, render and navigation for precision in surface continuity. Or?

It never really bothers me in appearance or measurement.

Perhaps wish list a user controlled conversion factor for NURBS objects?

Or an additional, ultra, super high option in the prefs?

-B