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peteb
09-13-2003, 06:35 AM
I've got a problem with Nurbs. It seems that when you extrude nurbs even in simple form, you get creases at the side. So for example if I make a simple grid made of quads, extrude a few in the middle and then convert to nurbs, I'll get a weird crease on the corners of where they were extruded. But then if I extrude the quads that I'd already extruded, the next set will convert fine.

Have a look at the pic and see what you think!

Mylenium
09-13-2003, 08:33 AM
Extrude? That would be wrong since extruding will not kill the original geometry. Due to this you get the sharp creases (multiple points from different polys converge). The only tools you should use with SubDs are bevel and Smooth Shift.

Mylenium

Mattoo
09-13-2003, 08:34 AM
Try using Smooth Shift for this rather than Extrude. Extrude leaves a the polygon you were selecting behind causing the problem you describe.

They're not NURBS either, it's Subpatches.

evenflcw
09-13-2003, 08:35 AM
It's because of the five polygon intersection and the step angle. Generally SubDs/subpatches/MetaNURBS may cause creasing or pinching with anything above or below 4 polygons connected to the same point. Normally 3 poly intersections aren't a problem, neither are 5 poly intersections, anything above 5 is something to definetly avoid. Too many 3 or 5 poly intersections too close to each are bad aswell.

What you could do to avoid these potentially bad intersections is use Spin Quads to change the flow or in this case weld some points together (see attachment;just make sure you delete the 2 point polygons/subpatches that might result from doing that).

Another thing to know and consider is that what you're seeing is Modelers realtime representation of (true?) subpatches and this differs from how subpatches are displayed in Layout AND how they are rendered. In Layout a subpatch mesh is both subdivided/"frozen" and tripled. So to check if a crease might cause problems where i counts (, in the render), either send the object to Layout and do a testrender, or just Freeze the mesh right in Modeler to get a quick peek if it works or not (make sure surface smoothing is on in the Surface Panel otherwise it'll look facetted).

If you try a testrender or just freeze your example mesh in Modeler, you'll probably find that the creases arent' visible anymore.

evenflcw
09-13-2003, 08:35 AM
hehe, oops, forgot the image.

peteb
09-13-2003, 09:57 AM
Nice one evenflcw, it's been so long since I've used Lightwave I forgot about the freeze button. In my pic I should have put edges on so you could see the model more clearly..sorry about that. But yeah basically it was a grid of quads, smooth shifted a few times. The reason why I said Extruded is because we have an in house modelling package at our studio and the extrude does the same thing as smooth shift. But thanks for all the replies guys, it's good to know you get a quick response on this site.