# Converting some geometry to NURBs surface

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### #1 jim c

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Posted 17 January 2012 - 07:44 PM

I've got some geometry that I'm trying to convert to a NURBs curve. Normally when I've played with the Convert SOP and feed it a Grid SOP, it converts the grid from a polygon(s) into a single NURMbs surface, which is *exactly* what I want.
However when I feed in my geometry from disc, it converts it into a bunch of NURBs surfaces, 1 per polygon. I can't figure out what's causing this or how to fix it. The geo is mad of 52 polygons, equivalent to a Grid SOP set to 5 rows, 14 columns.

I thought the problem might be related to the point order, but even if I create a 5X14 grid, and then apply a sort SOP set to random points, and the napply the Convert, I still get 1 single NURBs surface. Any thoughts on how to fix this?

### #2 Macha

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Posted 17 January 2012 - 08:26 PM

I'm not sure this is possible. The reason you get a nice Nurbsurface if you use primitives such as grids and spheres or even teapots is because a friendly scientist/geek/somebody took the time to work it out for us.

If you throw any geometry at it chances are there is no clear way to parametrize it. You'd essentially have to find out how to split the thing into a minimum amount of curves and I'm not sure such an algorithm even exists (a stable one). So the next best thing is to use each polygon and make them a nurbs(urface)

Edited by Macha, 17 January 2012 - 08:27 PM.

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### #3 LaidlawFX

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Posted 17 January 2012 - 09:42 PM

It would be best if you want nurbs in houdini you should start with nurbs in your other package. If your bring this in form max or maya they have a lot of tools to do nurbs modeling, and if you can't convert it to nurbs in those programs in a standard method, then you probably can't do it in a standard method in Houdini. But depending on what you're common shapes are you may be able to find a creative method in houdini like converting those rows in to separate lines and then skining them.

Also depending on why you want to convert this to nurbs you could probably find an alternative tool to do the job that is not a nurbs based tool. For instance on the former, if you just want it to look smooth at render time try just rendering the shape as sub-ds.
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### #4 kubabuk

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Posted 18 January 2012 - 10:27 AM

On approach would be to rebuilt your geometry as a nurbs surface using profile curves extracted from the polymesh edges. (see the example).

### #5 jim c

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Posted 18 January 2012 - 11:09 AM

Thanks guys! Actually I need/want to convert it so I can use it with a Creep SOP. I was fooling around the other day and came up with a nifty little network that makes eyelashes for characters. I already had these bits of geometry around for my other character that used a texture map for the eyelashes. I was hoping to try the creep technique with this geo.

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