NURBS surface hatch

[Stuart Allen]

I'm new to CAD software, using VW 11 on a MAC to model sculpture prior to fabrication. Using NURBS surfaces, I've successfully modeled some sail shapes, but I'd like to apply a hatch pattern to these surfaces. Is this possible?

thanks,

Stuart

[Biplab]

There is no direct functionality for this. An work around may be to use the contour tool to produce hatches.

[Stuart Allen]

that's a good tip, but I can't figure out a way to keep the contours from interacting poorly with the fill of the original nurbs surface. The contours fall partly above and partly below the original surface and a strange pattern emerges. any thoughts?

[propstuff]

If the contour tool is supposed to be producing contours of the surface, shouldn't they lie *exactly* on the surface? not a bit above and below. If they are above and below, then they're not actually contours are they?

N.

[Biplab]

Contours do lie on the surface within a reasonable tolerance. The problem is that when you render the surface, the surface is tessellated (tringulated) - which approximates curvy/wavy surface with planar triangles - thus the facets are slightly above or below the actual surface - hiding portions of the contour lines.

[Stuart Allen]

yes, that sounds like a good explanation. any ideas on how to accomplish my goal?

thanks for the help.

[Biplab]

This is the explanation - I am not making it up. In any case, I could not think of any solution for this problem. If your view is fixed, then you could convert the NURBS curves to polygons and then increase the thickness of the polygons to make them float over the surface rendering.

[propstuff]

2 things:

Biplab; what does"reasonable tolerance" mean?

If we were talking about, for instance, a casing for a mobile phone, 0.01mm would be a "reasonable" tolerance, but for the die-makers 0.1 would probably not.

In this example what tolerance would VW achieve in respect of the fidelity of contours of a NURBS surface?

Stuart; If you can make the countours and move them towards your view point by a small amount you may get clear of the surface facets, but not to the extent of the contours being noticibly "disconnected" from the object. (depends on your line thicknesses etc)

N.

[Biplab]

VectorWorks never works with absolute tolerances. In general the tolerance is ~= length of the diagonal of the bounding cube of the object * 1E-5 - which is quite low. For a die with width and breadth and height = 1m , bounding cube = 1m^3 - the tolerance is going to be 1.732 * 1E-5 ~= .02 mm.

Hope this gives you an idea.

[propstuff]

hence for my phone example, -say 100mm in length, the tolerance would be about 0.002mm.

Not exactly Aerospace grade engineering, but plenty good enough for a toaster (or a phone) :-)

cheers.

N.

[ 02-26-2005, 04:17 PM: Message edited by: propstuff ]