"Ben Chambers" <bdc### [at] junocom> wrote in
news:3eba9f93@news.povray.org: 

> 
> "Warp" <war### [at] tagpovrayorg> wrote in message
> news:3eb6d6b7@news.povray.org...
>>   I know next to nothing about NURBS surfaces, but as far as I
>>   understand, 
>> they are kind of extended bicubic patches (ie. they have everything
>> bicubic patches have plus lots of extra control parameters to tune
>> the surface shape). Bicubic patches are extremely versatile, but they
>> have their limits, which AFAIK NURBS don't have (or at least they
>> have less limits).
> 
> NURBS are just Non-Uniform Rational B-Splines.  Basically, they ARE
> bicubic patches (although the term is usually applied to a "sheet" of
> patches which are connected at the seems and of which only the actual
> corner points, not the control points, are manipulated).

AFAIK NURBS surface patches are not limited to being bicubic.
Their rational polynomials can be of any degree.

When you say that only the corner "points" are manipulated
when NURBS patches are stitched together, are you then
thinking of patches made with open knot vectors ?


> Unfortunately, you cannot model every shape with a spline of any type
> (that I am aware of).  Just try doing a sphere, and you'll see the
> problem :) 


I have an idea of how a sphere might be done with a single
NURBS patch, but I have not had time to try it yet.


Tor Olav

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