Is there one in MegaPov? if not, how hard would it be to add it?

Dave.
mic### [at] sympaticoca

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David Curtis wrote:
> 
> Is there one in MegaPov?

There is not.

> if not, how hard would it be to add it?

Techically it is possible. The question you need to ask is how useful
would it be. NURBS are vary handy in modelling programs for defining
and manipulating complex surfaces. Since POV-Ray is not a modelling
program there will be no way for you to directly change a NURBS surface
and it's usefulness becomes dubious. Triangles can and do accurately
represent the surfaces that NURBS do. Most any capable program that
can model NURBS surfaces can also exported to a format that can be
converted to a triangle based mesh.

-- 
Ken Tyler -  1300+ Povray, Graphics, 3D Rendering, and Raytracing Links:
http://home.pacbell.net/tylereng/index.html http://www.povray.org/links/

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In article <38C06E3A.9D90DCC4@pacbell.net>, lin### [at] povrayorg wrote:
>> if not, how hard would it be to add it?
>
>Techically it is possible. The question you need to ask is how useful
>would it be.

Sorry Ken, IMO that sounds like saying "why put in airbags when we already 
have seatbelts."

> NURBS are vary handy in modelling programs for defining
>and manipulating complex surfaces. Since POV-Ray is not a modelling
>program there will be no way for you to directly change a NURBS surface
>and it's usefulness becomes dubious.

I wouldn't use POV to change the surface just to render it.

> Triangles can and do accurately
>represent the surfaces that NURBS do. Most any capable program that
>can model NURBS surfaces can also exported to a format that can be
>converted to a triangle based mesh.

The program I use does convert the surface to triangles(and exports to POV 
with smooth triangles!) But, in the conversion there is always lost accuracy. 
When I render a converted mesh on the edges there is always straight lines 
connecting the vertices. This is my problem with it, I don't want to see the 
straight lines. Yeah, I could probably crank up the triangle count to minimize 
the error but, NURBS are perfectly accurate and I'd much rather get to 
construction data into POV so it can calculate the surface. Not to mention the 
extreme parse time you have to endure with thousands of triangles.

I checked out a program last night called Lightening3d. It exports NURBS to 
POV by converting to bezier patches. Sounds cool. Maybe this is the way that I 
could go?

Dave.
mic### [at] sympaticoca

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Subject: 
             Re: NURBS primitive?
        Date: 
             Sat, 04 Mar 2000 03:13:51 -0800
       From: 
             Ken <tyl### [at] pacbellnet>
    Reply-To: 
             lin### [at] povrayorg
 Organization: 
             Tyler Engineering Services
 Newsgroups: 
             povray.unofficial.patches
  References: 
             1 , 2 , 3





David Curtis wrote:
> 
> In article <38C06E3A.9D90DCC4@pacbell.net>, lin### [at] povrayorg wrote:
> >> if not, how hard would it be to add it?
> >
> >Techically it is possible. The question you need to ask is how useful
> >would it be.
> 
> Sorry Ken, IMO that sounds like saying "why put in airbags when we already
> have seatbelts."

I have heard many arguments for and against and so far the consensus seems
to favour not including them in POV-Ray. Yours is the first I have heard
where better quality is mentioned. Most argue in favour of smaller file
sizes than their triangle counterparts.

> I checked out a program last night called Lightening3d. It exports NURBS to
> POV by converting to bezier patches. Sounds cool. Maybe this is the way that I
> could go?

Patches have the advantage that you can change the smoothing of the patch
by changing a single parameter. The drawbacks to this is the amount of parse
time needed. The greater the amount of tessellation of the patch needed to
achieve the desired smoothness will add to the time it takes to parse each
patch. File sizes may tend to be a bit smaller but the trade off in file
size will be parsing time.

 For what it is worth I have rendered some enormous mesh files with as
many as a million triangles before and the the parsing time was not in
my opinion all that prohibitive. I have had while loops with much fewer
objects take considerably longer than a huge triangle mesh file takes
to parse and the memory requirement for while loops are much greater.

-- 
Ken Tyler -  1300+ Povray, Graphics, 3D Rendering, and Raytracing Links:
http://home.pacbell.net/tylereng/index.html http://www.povray.org/links/

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In article <38C0F1DD.26738120@pacbell.net>, lin### [at] povrayorg wrote:
>I have heard many arguments for and against and so far the consensus seems
>to favour not including them in POV-Ray. Yours is the first I have heard
>where better quality is mentioned. Most argue in favour of smaller file
>sizes than their triangle counterparts.
I take that into consideration aswell.

>Patches have the advantage that you can change the smoothing of the patch
>by changing a single parameter. The drawbacks to this is the amount of parse
>time needed. The greater the amount of tessellation of the patch needed to
>achieve the desired smoothness will add to the time it takes to parse each
>patch. File sizes may tend to be a bit smaller but the trade off in file
>size will be parsing time.
Really? Though I've never tried it (I probably shouldn't comment) I thought 
that with less vertices, less parse time.

>
> For what it is worth I have rendered some enormous mesh files with as
>many as a million triangles before and the the parsing time was not in
>my opinion all that prohibitive.
Agreed. It's the conversion from NURBS to triangles that's a killer for me.

So, I think i'll poke around a bit and confirm wether that it's too big a 
project for me.

Dave.
mic### [at] sympaticoca

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On Sat, 04 Mar 2000 10:15:13 GMT spa### [at] junkmailcom (David Curtis)
wrote:

>The program I use does convert the surface to triangles(and exports to POV 
>with smooth triangles!)

  I think Rhino 3d converts NURBS to triangles when exporting to
POV-Ray, also.

-- 
Alan - ako### [at] povrayorg - a k o n g <at> p o v r a y <dot> o r g
http://www.povray.org - Home of the Persistence of Vision Ray Tracer

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Alan Kong wrote:
> 
> On Sat, 04 Mar 2000 10:15:13 GMT spa### [at] junkmailcom (David Curtis)
> wrote:
> 
> >The program I use does convert the surface to triangles(and exports to POV
> >with smooth triangles!)
> 
>   I think Rhino 3d converts NURBS to triangles when exporting to
> POV-Ray, also.

True. Simon de Vet uses Rhino quite a bit these days and it's output
is very good even in mesh format. I am sure it has a lot to do with
the quality specified at export time as to how smooth the final object
will be.

-- 
Ken Tyler -  1300+ Povray, Graphics, 3D Rendering, and Raytracing Links:
http://home.pacbell.net/tylereng/index.html http://www.povray.org/links/

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Ken <tyl### [at] pacbellnet> wrote...
> I have heard many arguments for and against and so far the consensus seems
> to favour not including them in POV-Ray. Yours is the first I have heard
> where better quality is mentioned. Most argue in favour of smaller file
> sizes than their triangle counterparts.

NURBS, like subdivision surfaces, can be adaptively tesselated at
render-time.  This allows very high quality using very little memory at the
cost of slower render speeds.  Intelligent caching would probably help with
the speed problem.

-Nathan

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