
<html><head><meta http-equiv="content-type" content="text/html; charset=utf-8"></head><body style="overflow-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;"><div>The specification of Extrusion has a section „<span style="caret-color: rgb(0, 0, 0); color: rgb(0, 0, 0);">Coincident spine points</span>“ which describes the case when there are coincident spine points and different orientation or scale values:</div><div><br></div><div>13.3.5.4.4 Coincident spine points</div><div><br></div><a href="https://www.web3d.org/specifications/X3Dv4Draft/ISO-IEC19775-1v4.1-CD/Part01/components/geometry3D.html#CoincidentSpinePoints">https://www.web3d.org/specifications/X3Dv4Draft/ISO-IEC19775-1v4.1-CD/Part01/components/geometry3D.html#CoincidentSpinePoints</a><br id="lineBreakAtBeginningOfMessage"><div>

<meta charset="UTF-8"><div dir="auto" style="text-align: start; text-indent: 0px; overflow-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;"><div dir="auto" style="text-align: start; text-indent: 0px; overflow-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;"><div dir="auto" style="text-align: start; text-indent: 0px; overflow-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;"><div dir="auto" style="text-align: start; text-indent: 0px; overflow-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;"><div dir="auto" style="text-align: start; text-indent: 0px; overflow-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;"><div dir="auto" style="text-align: start; text-indent: 0px; overflow-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;"><div dir="auto" style="text-align: start; text-indent: 0px; overflow-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;"><div dir="auto" style="text-align: start; text-indent: 0px; overflow-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;"><div dir="auto" style="text-align: start; text-indent: 0px; overflow-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;"><div dir="auto" style="text-align: start; text-indent: 0px; overflow-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;"><div dir="auto" style="text-align: start; text-indent: 0px; overflow-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;"><div><br></div><div><font color="#000000">And this is a test for this case for coincident spine points and only orientations, as Andreas already mentioned:</font></div><div><br></div><div><a href="https://create3000.github.io/x_ite/playground/?url=https://create3000.github.io/Library/Tests/Components/Extrusion/Rotations.x3d">https://create3000.github.io/x_ite/playground/?url=https://create3000.github.io/Library/Tests/Components/Extrusion/Rotations.x3d</a></div><div><br></div><div>Same with coincident spine points and only scales:</div><div><br></div><div><a href="https://create3000.github.io/x_ite/playground/?url=https://create3000.github.io/Library/Tests/Components/Extrusion/Scales.x3d">https://create3000.github.io/x_ite/playground/?url=https://create3000.github.io/Library/Tests/Components/Extrusion/Scales.x3d</a></div><div><br></div><div>Best regards,</div><div>Holger</div><div><br></div><div><font color="#000000"><span style="caret-color: rgb(0, 0, 0);">—</span></font></div><div style="caret-color: rgb(0, 0, 0); color: rgb(0, 0, 0); letter-spacing: normal; text-transform: none; white-space: normal; word-spacing: 0px; text-decoration: none; -webkit-text-stroke-width: 0px;">Holger Seelig<br>holger.seelig@yahoo.de<br><br></div></div></div></div></div></div></div></div></div></div></div></div>

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<div><br><blockquote type="cite"><div>Am 11.07.2025 um 07:11 schrieb Andreas Plesch via x3d-public <x3d-public@web3d.org>:</div><br class="Apple-interchange-newline"><div><div dir="ltr"><div>Extrusions are useful but have many edge cases.</div><div><br></div><div>Looking into how to improve a bit x3dom's implementation I came across:</div><div><br></div><div><a href="https://www.web3d.org/x3d/content/examples/ConformanceNist/Geometry/Extrusion/test_YZextrusionIndex.html">https://www.web3d.org/x3d/content/examples/ConformanceNist/Geometry/Extrusion/test_YZextrusionIndex.html</a></div><div><br></div><div>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.</div><div><br></div><div>I am wondering if adding this flip was unintentional since it is not mentioned in the description?</div><div><br></div><div>Removing the colinearity of these first spine points to isolate the effect of flipping direction results in:</div><div><br></div><div><a href="https://andreasplesch.github.io/Library/Viewer/index.html?url=data:text/plain;base64,PFgzRD4KICA8U2NlbmU+CiAgICAgIDxTaGFwZT4KICAgICAgICA8QXBwZWFyYW5jZT4KICAgICAgICAgIDxNYXRlcmlhbC8+CiAgICAgICAgPC9BcHBlYXJhbmNlPgogICAgICAgIDxFeHRydXNpb24gYmVnaW5DYXA9J2ZhbHNlJyBjcm9zc1NlY3Rpb249JzEgMCAwLjc1IDAuNjYgMC41IDAuODcgMC4yNSAwLjk3IDAgMSAtMC4yNSAwLjk3IC0wLjUgMC44NyAtMC43NSAwLjY2IC0xIDAgLTAuNzUgLTAuNjYgLTAuNSAtMC44NyAtMC4yNSAtMC45NyAwLjI1IC0wLjk3IDAuNSAtMC44NyAwLjc1IC0wLjY2IDEgMCcgZW5kQ2FwPSdmYWxzZScgc29saWQ9J2ZhbHNlJyBzcGluZT0nMCAwIC0xLCAwIDAuMDEgLTIsIDAgMCAwJy8+CiAgICAgIDwvU2hhcGU+CiAgICAgIDwvU2NlbmU+CjwvWDNEPg==">https://andreasplesch.github.io/Library/Viewer/index.html?url=data:text/plain;base64,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</a></div><div><br></div><div>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.</div><div><br></div><div>Another, not well defined but potentially useful edge case is a spine with only coincident points:</div><div><br></div><div><a href="https://create3000.github.io/x_ite/playground/?url=https://raw.githubusercontent.com/create3000/Library/8f64c4caf4fcc9fc9e429d213caef2feba008efb/Tests/Components/Geometry3D/Extrusion/Rotations.x3d">https://create3000.github.io/x_ite/playground/?url=https://raw.githubusercontent.com/create3000/Library/8f64c4caf4fcc9fc9e429d213caef2feba008efb/Tests/Components/Geometry3D/Extrusion/Rotations.x3d</a></div><div><br></div><div>The extrusions are solely based on changes in orientations.</div><div><br></div><div>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 ?</div><div><br></div><div>Andreas</div><span class="gmail_signature_prefix">-- </span><br><div dir="ltr" class="gmail_signature" data-smartmail="gmail_signature"><div dir="ltr"><div>Andreas Plesch<br>Waltham, MA 02453</div></div></div></div>

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