[X3D-Public] reason / benefits of homogeneous coordinates in NURBS?

[Philipp Serfling]

Thx to all of you for your great help.

 

Considering this reasons, i will stick with the homogeneous way.

 

.philipp

 

Von: Keith Victor [mailto:keithrvictor at gmail.com] 
Gesendet: Dienstag, 27. August 2013 06:58
An: Philipp Serfling
Cc: x3d-public at web3d.org
Betreff: Re: [X3D-Public] reason / benefits of homogeneous coordinates in
NURBS?

 

Hello Philipp:

Yes, I feel your pain.

I implemented X3D NURBS in Vivaty Player/Vivaty Studio many years ago, and
recall running into that problem.

The spec clearly says that fourth parameter is "weight", and references the
NURBS book, that tells you exactly how to implement it.  The Spec and the
NURBS book do NOT refer to "weight" as being "w", the forth homogenous
coordinate.  weight is weight.  It does however, start with the letter "w".



But, Contact and others used the "weight" as "w", the fourth homogenous
coordinate.  Since that is the gold standard, the bug propagates. 


FWIW, Vivaty used the proper interpretation of the NURBS weight field  :)

 

So, to answer your question, IMHO, the only benefit of choosing the
Homogenous coordinate interpretation is that it is consistent with the other
X3D players.  Consistent with the spec... not so much.

 

Keith Victor

Apple

(these are my opinions, and not those of my employer)

 

 





 

On Mon, Aug 26, 2013 at 12:04 AM, Philipp Serfling
<Philipp.Serfling at t-systems-mms.com> wrote:

Hi,
i´m sorry to bother you again with my problems, but if anyone know some
resources or reasons for my problem written underneath, i would be very
thankful.

thx
.philipp


-----Ursprüngliche Nachricht-----
Von: Philipp Serfling [mailto:Philipp.Serfling at t-systems-mms.com]
Gesendet: Mittwoch, 14. August 2013 15:09
An: 'x3d-public at web3d.org'
Betreff: reason / benefits of homogeneous coordinates in NURBS?

hello,

I´ve noticed that in x3d the control points of a nurbs surface are
represented in homogeneous coordinates
see:
http://castle-engine.sourceforge.net/x3d_implementation_nurbs.php#section_ho
mogeneous_coordinates

Is there any benefit in doing it that way compared to the "normal /
intuitive way" (also explained in the link) ?

I´m asking for two reasons:

1. White_dune, Instant Reality & BSContact  Player are using the homogeneous
way.
    But Octaga Player uses the "normal" way.

2. I´m currently implementing NURBS Curves and PatchSurfaces in X3DOM and
i´m not sure if i should implement the homogeneous
  or the "normal way". Currently i´m developing both, but the homogeneous
way is unintuitive and more complex and i really don´t see any advance.

thx for your help
.philipp


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