Creating Surface from NURBS Problems

[Jake DeGroot]

Hi,

I am trying to create the top of a tent in 3D. I draw each surface of the top in 2D, from a top view. Then I converted those shapes to NURBS curves. From there, I was able to adjust the Z-value of each vertex, to get the outline perfect. I have attached a v2010 file at this stage, with 6 NURBS curves all aligned to form the top of the tent, stretched over the tent poles.

The problems appear when I start converting these closed curves to surfaces. The top two curves convert just as I would expect. Perfect. The short sides, are almost right, but the bottom corners get extended out to the sides, making them somewhat inaccurate. The long sides are completely wrong when I convert them. The resulting surface has a huge protruding puckering shape and I don't know where it comes from or how to get rid of it.

What am I doing wrong? What do I have to do to these curves to make them convert cleanly into surfaces as I would expect? Or is there a better method to use to draw these shapes?

Thanks so much!

Jake

[Kevin McAllister]

Hi Jake,

I suspect its the little wings I've highlighted in red that are causing you problems. Your two top surfaces drape as you expect a real tent to drape and so they behave properly. The sides don't quite drape as a real tent would and so they aren't behaving well (they bulge at the centre where the "wings" are and this is causing the deformation when you convert the curves to surfaces.

When you create the tent in plan view, you've curved the lines in the xy direction. If you keep them straight, like the lines I've drawn in red, you will have more success I think.

Kevin

[Benson Shaw]

Jake

I don?t think you can get the whole tent as one surface, if that's what you were after. Kevin is right, those out of plane curves were making gnarly surfaces.

I made half a tent and mirrored it. I assumed sag in edges is z direction only - no deflection to the sides. Started with flat (but tilted) NURBS surfaces, and adjust z values of vertices. Edges appear as straight lines in plan.

1. Draw half of one of the long tent surfaces

In Top/Plan, draw a NURBS curve (interpolation point mode) over your orig tent for half of a long side. 4 points: pole top, pole top, corner, long side base mid point, back to start. Or, draw a poly, convert to NURBS, adjust z

2. Shape the half side

3d PowerPack>Create Surface from Curves, set degree 3 for U and V .

Set z value of top midpoint to 32? or what ever you like.

Decrease the z value of the 3 horizontal mid points to give a little sag to the face. Or increase the OIP weight value.

3. Mirror the half side

Top/Plan>Mirror tool on tent long axis

4. Make the end face

Off axis view. Extract tool>ShiftClick end edge on each half side, checkmark

Ungroup & loft the two new NURBS curves - ruled mode

Lower z of upper mid point

5. Make a top for this half tent

Extract top edges of the two sides, Loft (ruled). Adjust z for mid points. Sometimes it helps to 3d PowerPack>Create Interpolated surface - this puts the vertices on the surface.

6. Mirror the half tent

Top/Plan, Select the 4 surfaces, Mirror on tent short axis.

HTH

-B

[Kaare Baekgaard]

Hello Jake

If you use that tent outdoors, you will have problems, if it starts to rain.

I propose a different approach, which should be fast, easy and foolproof:

Start with an extruded rectangle. It is best to do a quarter- or half tent and then mirror it up to the full shape.

Use solids or nurbs surfaces to carve away the sides by sectioning or subtraction.

Use a sphere to carve the top away.

Mirror the shapes and add solid them.

Then shell the resulting shape from the bottom.

The good thing about this method is, that you keep the history of the tent intact. Changes can be applied easily by working with the sectioning surfaces.

The method was used for this chair, which still has an intact history. Hundreds of changes were made in the design phase.

Edited August 24, 2011 by Kaare Baekgaard