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

[Andreas Plesch]

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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