[x3d-public] Nurbs control points, preweighted or not ?

[Andreas Plesch]

Thanks, Michalis, for the quick confirmation. Should we use X3D v4.0 to
change the undesired behavior ? v4.0 allows for backward incompatiblity.

If the issue is not significant enough to break backward compatibility,
there should be a sentence explaining the issue: "Control points are
provided as homogeneous coordinates, eg. multiplied by their corresponding
weights." or something like that.

I can provide a spec comment if that is helpful, eg. if Michalis did not
already provide one.

Cheers, -Andreas

On Tue, May 19, 2020 at 9:50 AM Michalis Kamburelis <
michalis.kambi at gmail.com> wrote:

> See the documentation about this issue on Castle Game Engine:
>
> https://castle-engine.io/x3d_implementation_nurbs.php#section_homogeneous_coordinates
>
> As far as I tested, all X3D browsers require points to be
> "premultiplied by the given weight". Which is confusing and contrary
> to how NURBS are usually expressed (it makes the operation
> "intuitively pull the curve toward the control point" harder than it
> should be, see my docs above), but that's how it is. View3dscene /
> Castle Game Engine follow it.
>
> I have some simple tests of NURBS in
> https://github.com/castle-engine/demo-models/tree/master/nurbs .
>
> Regards,
> Michalis
>
> wt., 19 maj 2020 o 15:28 Andreas Plesch <andreasplesch at gmail.com>
> napisał(a):
> >
> > We came across a specification issue for Nurbs. It is unclear if
> provided control points should be provided
> >
> > A) already weighted, eg. premultiplied by the given weight, or
> > B) if the browser, eg. the evaluation of the Nurbs should do the
> weighting, eg. the multiplication of each control point with the
> corresponding weight.
> >
> > At first glance, since the weights are provided separately, one would
> think the control points should be provided unweighted, eg. B). This is
> also how OpenCascade or Blender specify control points (
> https://docs.blender.org/manual/en/latest/modeling/surfaces/structure.html
> ).
> >
> > But tests using a torus Nurbs reveal all freeWrl, view3dscene and
> InstandPlayer expect premultiplied control points (case A). This is
> surprising but there may be a reason for that consistency. Perhaps a
> commonly used formula ?
> >
> > The spec. itself is silent on this.
> >
> > A) requires confusing redundancy, since the weight field is still needed
> for normalizing the weighted sum.
> >
> > I would prefer B).
> >
> > There are no examples which use important weights, for testing. The
> github issue below has the torus example.
> >
> > If A) is the consensus, the spec. needs to point out that control points
> are expected to be already weighted.
> >
> > Here is the related discussion:
> > https://github.com/x3dom/x3dom/issues/1059
> > https://github.com/andreasplesch/OCCToX3D/issues/8
> >
> >
> >  -Andreas
> >
> > --
> > Andreas Plesch
> > Waltham, MA 02453
> > _______________________________________________
> > x3d-public mailing list
> > x3d-public at web3d.org
> > http://web3d.org/mailman/listinfo/x3d-public_web3d.org
>


-- 
Andreas Plesch
Waltham, MA 02453
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