[x3d-public] Extrusions, take 3

[Andreas Plesch]

https://github.com/x3dom/x3dom/issues/570

has some discussion on x3dom extrusion deficiencies with graphics. However,
these are not the result of the spec. not spelling out corner cases but of
bugs.

A use case for Extrusion for me was easy construction of helical gears from
cross-sections. I had first constructed those as IFS but not having to deal
with triangulation was a big plus for Extrusion when generated by an
algorithm.

Should the spec. make explicit if the cross-sections along the spine should
be connected as quads or as triangles ? Both options present themselves.
Preferring or requiring one could lead to better fidelity across browsers.

As long as the number of vertices in the cross-section or in the spine is
not modified during an animation, there should be no need for
reindexing/retriangulation. A smart browser could just update affected
vertex positions in the existing mesh. However, the same is true for
animation of IFSs.

Andreas
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