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

[Michalis Kamburelis]

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
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