[x3d-public] bilinear and coincident Extrusion spines

[Holger Seelig]

The specification of Extrusion has a section „Coincident spine points“ which describes the case when there are coincident spine points and different orientation or scale values:

13.3.5.4.4 Coincident spine points

https://www.web3d.org/specifications/X3Dv4Draft/ISO-IEC19775-1v4.1-CD/Part01/components/geometry3D.html#CoincidentSpinePoints

And this is a test for this case for coincident spine points and only orientations, as Andreas already mentioned:

https://create3000.github.io/x_ite/playground/?url=https://create3000.github.io/Library/Tests/Components/Extrusion/Rotations.x3d

Same with coincident spine points and only scales:

https://create3000.github.io/x_ite/playground/?url=https://create3000.github.io/Library/Tests/Components/Extrusion/Scales.x3d

Best regards,
Holger

—
Holger Seelig
holger.seelig at yahoo.de


> Am 11.07.2025 um 07:11 schrieb Andreas Plesch via x3d-public <x3d-public at web3d.org>:
> 
> Extrusions are useful but have many edge cases.
> 
> Looking into how to improve a bit x3dom's implementation I came across:
> 
> https://www.web3d.org/x3d/content/examples/ConformanceNist/Geometry/Extrusion/test_YZextrusionIndex.html
> 
> The beginning of the spine here has a weird 180 degree flip in direction, from -1 to -2 back to 0 on the y axis.
> 
> I am wondering if adding this flip was unintentional since it is not mentioned in the description?
> 
> Removing the colinearity of these first spine points to isolate the effect of flipping direction results in:
> 
> https://andreasplesch.github.io/Library/Viewer/index.html?url=data:text/plain;base64,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
> 
> x_ite interpretes this differently. Extreme spine direction reversals are probably out of spec. scope so I think the Nist Conformance example may have a typo.
> 
> Another, not well defined but potentially useful edge case is a spine with only coincident points:
> 
> https://create3000.github.io/x_ite/playground/?url=https://raw.githubusercontent.com/create3000/Library/8f64c4caf4fcc9fc9e429d213caef2feba008efb/Tests/Components/Geometry3D/Extrusion/Rotations.x3d
> 
> The extrusions are solely based on changes in orientations.
> 
> Since there is no way to define a unique SCP, these extrusions are undefined. It may be worth defining this edge case explicitly by declaring the SCP axes to align with the local coordinate system. But I think this would require a spec. amendment. Would it be worth considering ?
> 
> Andreas
> --
> Andreas Plesch
> Waltham, MA 02453
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