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

Keith Victor keithrvictor at gmail.com
Mon Aug 26 21:57:42 PDT 2013

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

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
(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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