[x3d-public] Problem constructing Mobius strip by extrusion of rectangular cross-section

[vmarchetti at kshell.com]

> Tony
> 
> I see from your snippets that the cross section is a rectangle; in the XZ plane,
> width of 1 in X axis and 0.1 in the Z axis; and the geometric center of this rectangle is at
> x,z = (0.5, 0.95). The Extrusion spec at https://www.web3d.org/documents/specifications/19775-1/V4.0/index.html states
> 
> "For example, to impart twist in the cross-section, a rotation about the local Y-axis (0 1 0) would be used"
> 
> So in your model you are not twisting the cross section about its geometric center. After one spin around the spine in the mobius case
> the cross section will be centered at (-0.5, -0.95) so will not, as you obsere line up. 
> 
> I believe the solution will be to restate your cross section so that it is centered, and symmetric about the (0,0,0) origin, and in the XZ plane
> 
> Vince Marchetti
> 

Looking at your screenshots (your 3rd entry to this thread) makes me more confident that this is the case.
I look forward to seeing the corrected results.

And your scene at https://www.laetusinpraesens.org/docs10s/images/tenperc_files/mobius_borro_three.x3d is impressive as well

Vince Marchetti

> On Aug 28, 2023, at 7:44 AM, Anthony Judge <anthony.judge at gmail.com> wrote:
> 
> Enclosed are the Extrusion snippets for both models
> 
> Tony
> 
> On 28/08/2023, Anthony Judge <anthony.judge at gmail.com> wrote:
>> Just to be clear, I have successfully built models with the Mobius
>> strip, as indicated in the following
>> 
>> https://www.laetusinpraesens.org/docs10s/images/tenperc_files/mobius_borro_three.x3d
>> 
>> I enclose screen shots of the 2-twist and 1-twist version (thre latter
>> in wireframe)
>> 
>> Tony
>> 
>> On 28/08/2023, Anthony Judge <anthony.judge at gmail.com> wrote:
>>> Hi Vince
>>> 
>>> Thanks for the interest.
>>> 
>>> I could easily  share screen shots but the matter is so simple that
>>> sharing the Extrusion of a 2-twist 360 orientation along the spine,
>>> with the Extrusion of a 1-twist 180 orientation might be simplest? It
>>> is the latter which fails and the former which is correct, but not
>>> what I want.
>>> 
>>> My thought is the difficulty is the manner in which the Extrusion
>>> relates the orientation to the spine. For the 180 degree case, what is
>>> effectively required is that the spine (or a "fake spine") follow the
>>> center of the rotating rectangular cross-section. Of course it does
>>> not. It follows one edge. So 360 connects up but 180 ends up offset.
>>> 
>>> Of course I may have messed up with the parameters of the Extrusion
>>> 
>>> Do you need the screen shots?
>>> 
>>> Tony
>>> 
>>> On 28/08/2023, vmarchetti at kshell.com <vmarchetti at kshell.com> wrote:
>>>> Tony
>>>> 
>>>> This sounds like an interesting project for the X3D community to help
>>>> with.
>>>> 
>>>> As a proof of concept, this X-ITE rendering shows it is possible to
>>>> render
>>>> Mobius-like geometry in X3D:
>>>> https://spri-open-resources.s3.us-east-2.amazonaws.com/trimobius/index.html
>>>> 
>>>> 
>>>> The X3D file that this page is rendering may be retrieved at
>>>> https://spri-open-resources.s3.us-east-2.amazonaws.com/trimobius/mobius.x3d
>>>> 
>>>> If you can share screenshots or webpages with your not-satisfactory
>>>> results,
>>>> that may give us some hint as to what's going on, and also sharing code
>>>> may
>>>> also help us.
>>>> 
>>>> Vince Marchettti
>>>> Web3D Consortium
>>>> 
>>>> 
>>>> 
>>>>> On Aug 28, 2023, at 1:38 AM, Anthony Judge <anthony.judge at gmail.com>
>>>>> wrote:
>>>>> 
>>>>> I have a working model of a Mobius strip, but it uses Indexed Faceset
>>>>> which I cannot manipulate as I had hoped to do. Basically I want to
>>>>> extend the edges of the strip, possibly dynamically.
>>>>> 
>>>>> So as an alternative I extruded a thin rectangular cross-section along
>>>>> a circular spine, but modifying the orientation of each element along
>>>>> the spine through 180 degrees.
>>>>> 
>>>>> This gives a correct result, except that the ends do not meet
>>>>> correctly -- the end is offset because it is rotated around the spine.
>>>>> This is apparent if I change the orientation through 360 degrees, when
>>>>> the ends do connect correctly -- but I have two twists in the strip,
>>>>> which is not what I need for a Mobius strip
>>>>> 
>>>>> I can provide snippets of this simple issue and I can understand how
>>>>> the issue is created by using the spine/orientation combination with
>>>>> the extrusion.
>>>>> 
>>>>> Any comments/help?
>>>>> 
>>>>> Tony
>>>>> 
>>>>> --
>>>>> Anthony Judge
>>>>> <anthony.judge at gmail.com>
>>>>> www.laetusinpraesens.org
>>>>> 
>>>>> _______________________________________________
>>>>> x3d-public mailing list
>>>>> x3d-public at web3d.org
>>>>> http://web3d.org/mailman/listinfo/x3d-public_web3d.org
>>>> 
>>>> 
>>> 
>>> 
>>> --
>>> Anthony Judge
>>> <anthony.judge at gmail.com>
>>> www.laetusinpraesens.org
>>> https://en.wikipedia.org/wiki/Anthony_Judge
>>> 
>> 
>> 
>> --
>> Anthony Judge
>> <anthony.judge at gmail.com>
>> www.laetusinpraesens.org
>> https://en.wikipedia.org/wiki/Anthony_Judge
>> 
> 
> 
> -- 
> Anthony Judge
> <anthony.judge at gmail.com>
> www.laetusinpraesens.org
> https://en.wikipedia.org/wiki/Anthony_Judge
> <mobius_180_1_twist.txt><mobius_360_2_twist.txt>


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