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

Michalis Kamburelis michalis.kambi at gmail.com
Tue Aug 27 15:22:48 PDT 2013

Keith Victor wrote:
> 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.

Same thing with NURBS implementation in view3dscene (done based on White 
Dune implementation). Using the homogenous coordinates convention for 
controlPoint felt a little weird, and it doesn't make any noticeable 
speed gain, but it had to be implemented like that for compatibility 
with other browsers. It's not a big problem, but you have to remember 
about it.

Like Vincent notes, at least all implementations are consistent about 
it. And it would be good to mention this in the X3D spec.


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