[x3d-public] Extrusion texture coordinates

[Don Brutzman]

Thanks for noticing this Andreas.

To answer your question generally, the functionality of the specification
is expected to be unambiguous.

   - X3D Architecture version 4.1 draft, clause 13 Geometry3D component,
   13.3.5 Extrusion
   -
   https://www.web3d.org/specifications/X3Dv4Draft/ISO-IEC19775-1v4.1-CD/Part01/components/geometry3D.html#Extrusion

To answer specifically, I believe we have always thought of *relative *distance
along the spine as the fraction [0..1] and that is consistent with other
u-v-w type mappings in the X3D specification.

What do you think of this suggested addition after the "Textures are
mapped..." sentence that you have quoted above:

   - "Relative distances from beginning to end along the *crossSection
*and *spine
   *curves are used to compute values in the range [0..1]."

Note that there might only be multiple coincident points for a *spine *vection,
permitting rotations of the *crossSection* according to the orientations
field about that single point.

As another potential special case, let's consider all *crossSection *points
to be coincident.  Am not seeing anything in the specification forbidding
that.  For a rendered result, am expecting to see a single line connecting
each of those single-point *crossSection *perimeters, i.e. a line made up
of segment(s) that gets extruded at an appropriate distance from the spine.

Editorially, I think we ought to be wary about defining forbidden
ill-defined cases.  Such definitions can be unexpectedly ambiguous, and
special handling (from forbidden status) might detract unnecessarily from
Extrusion animations.

Of note is that we do have a number of example Extrusion nodes intended to
test baseline capabilities and special cases.  If you or anyone thinks we
should add more cases, please advise (single-point *crossSection *perhaps).

   - X3D Example Archives: X3D4AM, X3D for Advanced Modeling, Geometric
   Shapes, Extrusion Examples Test
   -
   https://www.web3d.org/x3d/content/examples/X3dForAdvancedModeling/GeometricShapes/ExtrusionExamplesTestIndex.html

Thanks for noting and considering a possible specification improvement.
Have fun with X3D Extrusion!  😀

all the best, Don


On Sat, Jul 12, 2025 at 11:40 PM Andreas Plesch via x3d-public <
x3d-public at web3d.org> wrote:

> Looking into how to generate texture coordinates for Extrusions with all
> coincident spine points, I noticed this section in the spec.:
>
> "Textures are mapped so that the coordinates range in the U direction from
> 0 to 1 along the crossSection curve (with 0 corresponding to the first
> point in crossSection and 1 to the last) and in the V direction from 0 to 1
> along the spine curve (with 0 corresponding to the first listed spine point
> and 1 to the last)."
>
> V varies along the spine from 0 to 1. The spec. does not define how V
> should vary. x3dom uses the distance between spine points to define V.
> Another option may be to use the index of a spine point to define V. There
> may be other strategies.
>
> Both options produce similar results if the spine points are regularly
> spaced. However, in the case of a spine with all coincident points, the
> results will be very different. Using distance, all Vs will be 0 (except
> for last to satisfy the spec.). Using the index, Vs will be distributed.
>
> Should the spec. better define how Extrusion UVs should vary ?
>
> Cheers, -Andreas
>
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