[x3d-public] bilinear and coincident Extrusion spines

[Andreas Plesch]

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
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://web3d.org/pipermail/x3d-public_web3d.org/attachments/20250711/78790ebf/attachment.html>