POV-Ray : Newsgroups : povray.tools.general : NURBS Server Time: 30 Mar 2020 14:16:28 GMT
  NURBS (Message 1 to 4 of 4)  
From: Bald Eagle
Subject: NURBS
Date: 5 Mar 2020 00:15:01
Message: <web.5e604408822739b24eec112d0@news.povray.org>
I've been looking into doing some sort of implementation of NURBS recently, and
came across this:

Open-source, cross-platform NURBS
http://verbnurbs.com/

Haven't had the opportunity to DL or install...


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From: Le Forgeron
Subject: Re: NURBS
Date: 11 Mar 2020 20:44:44
Message: <5e694dbc$1@news.povray.org>
Le 05/03/2020 à 01:12, Bald Eagle a écrit :
> 
> I've been looking into doing some sort of implementation of NURBS recently, and
> came across this:
> 
> Open-source, cross-platform NURBS
> http://verbnurbs.com/
> 
> Haven't had the opportunity to DL or install...
> 
> 

If you are on Nurbs, I did them in hg-povray.

http://wiki.povray.org/content/User:Le_Forgeron/nurbs

It's just missing a modeler to export the raw data (all the modelers
I've seen export them as mesh, something easily done in SDL).


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From: Bald Eagle
Subject: Re: NURBS
Date: 11 Mar 2020 21:25:01
Message: <web.5e6956746d1ce2044eec112d0@news.povray.org>
Le_Forgeron <jgr### [at] freefr> wrote:

> If you are on Nurbs, I did them in hg-povray.
> http://wiki.povray.org/content/User:Le_Forgeron/nurbs

I knew you'd tackle them eventually.   It must have been virtually irresistible
for you.  :D

> It's just missing a modeler to export the raw data (all the modelers
> I've seen export them as mesh, something easily done in SDL).

Well, I had thought about trying to do something with NURBS in Javascript, as a
project to spur me along the learning curve, and use as a qualification of some
sort.

From the "small" of amount of reading I've done, the big difference between
NURBS and Bezier are the basis functions - NURBS control points being much more
local in their effect on the curve. I'm going through Piegl's _The NURBS Book_ -
which is excellent - very thorough, and with code examples.
I'm assuming that for a good modeler, knot insertion, refinement, and removal
would be highly desirable.

I saw a while back that you had started on NURBS, and actually did rational
Bezier - that seemed easy.   Did you find it a real challenge to get proper
NURBS working correctly?

Thanks, Jerome  :)


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From: Le Forgeron
Subject: Re: NURBS
Date: 13 Mar 2020 20:17:16
Message: <5e6bea4c$1@news.povray.org>
Le 11/03/2020 à 22:21, Bald Eagle a écrit :
> 
> Le_Forgeron <jgr### [at] freefr> wrote:
> 
>> If you are on Nurbs, I did them in hg-povray.
>> http://wiki.povray.org/content/User:Le_Forgeron/nurbs
> 
> I knew you'd tackle them eventually.   It must have been virtually irresistible
> for you.  :D
> 
>> It's just missing a modeler to export the raw data (all the modelers
>> I've seen export them as mesh, something easily done in SDL).
> 
> Well, I had thought about trying to do something with NURBS in Javascript, as a
> project to spur me along the learning curve, and use as a qualification of some
> sort.
> 
> From the "small" of amount of reading I've done, the big difference between
> NURBS and Bezier are the basis functions - NURBS control points being much more
> local in their effect on the curve. I'm going through Piegl's _The NURBS Book_ -
> which is excellent - very thorough, and with code examples.
> I'm assuming that for a good modeler, knot insertion, refinement, and removal
> would be highly desirable.
> 
> I saw a while back that you had started on NURBS, and actually did rational
> Bezier - that seemed easy.   Did you find it a real challenge to get proper
> NURBS working correctly?

The main issue I remember was understanding knots and weights.

And when you look at most implementations, they are limited in the
supported order and often optimised (far too optimised) to compute mesh.

De Boor's is really your friend, and it was nice to still remember how
to compute derivation, at the formula level.

I had the luck to find a very comprehensive document (see code, it is
cited there (source/core/shape/nurbs.cpp) ).

The interesting point of rational bezier is that there is a solver to
compute the intersection with a ray, something that I give up on the
Nurbs due to the opened order (and knots!).

Nevertheless, you can also generate a mesh from the ration bezier
(trading speed against memory and accuracy of real formula)


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