[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 it 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 obseve line. 

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




> 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>