CGTalk - Convert a Linear NURBS surface to a cubic one

View Full Version : Convert a Linear NURBS surface to a cubic one

mihao

02-18-2003, 12:33 AM

I am in problem. Need help really. :hmm:

At the very beginning, I lofted a NURBS surface with linear degree, because the linear result have much less CVs that the cubic one. I through that I can convert this surface to a cubic one when I finish editing this surface.

Then I begin to do things like moving CVs, inserting isoparms on this NURBS surface.

Unfortunately, when I convert this surface into a cubic one by changing its construction histroy, that is, change loft degree from linear to cubic, terrible thing happens as following picture is depicting. :eek:

So How Can I Convert A Linear NURBS Surface Into A Cubic One Successfully?

luminis

02-18-2003, 02:51 AM

Go to 'Edit NURBS/Rebuild Surfaces []' and open its option box. Set the settings for it *exactly* as this picture:

http://www.luminis3d.com/etc/cgt/img/reb.jpg

Rebuild Type: Uniform
Parameter Range: 0 to #Spans
Direction: U and V
Keep: [x] CVs
Degree U: 3 Cubic
Degree V: 3 Cubic

Hope this helps :)

mihao

02-18-2003, 04:34 AM

Thanks. it works :thumbsup:

But can u give me some explaination on why this distortion happens when change linear to cubic?

Many thanks.
:rolleyes:

playmesumch00ns

02-18-2003, 10:24 AM

Its hard to explain this simply, but basically you don't want to do it like that! NURBS are complicated beasts!

Linear and cubic refers to the degree of the blending functions. You need 2 points to define a linear curve segment, 3 for a quadratic and 4 for a cubic.

So going back and trying to change the degree of the surface from the construction history will try and interpolate the same set of CVs with different blending functions, which means that each surface patch you made before will be subsumed into the new ones as you need four times as much geometry to define it.

I'm guessing also that all your insert isoparm and tweak nodes will also be screwed up by doing this.

The rebuild surfaces command, however, will take this into account and use some mathematical hocus-pocus to give you the results you expect.

Sorry if this explanation's a little muddy, but so's my head. When I get back to a copy of Maya, I'll try making some pictures to illustrate.

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