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# Geodesic curve offset

Member Posts: 19 ✭✭✭
edited March 2017
Requested feature summary: A geodesic offset of a given curve on a given surface at a given distance in a given direction.

Example application: Modeling of wind turbine blade composite layups, especially draping of fabrics, preventing detailed draping simulation. For an example and a possible workaround, see Figure 1. In Solidworks, the workaround often resulted in erroneous behavior (back in 2010, maybe it has improved).

Figure 1: (left) Example of a geodesic offset of curve
C on surface S of a wind turbine blade, yielding geodesic offset curve O. (right) An approximation using multiple swept surfaces. (bottom) Mathematical expression, coarsely defining a geodesic offset curve. Note that the path of arc length d is generally not coplanar.

Putting the sketchy math of Figure 1 in words: Curve O is a `geodesic Offset curve´ from curve C at arc length d, if, for each point q on curve O, there exists at least one path p on surface S from point q to curve C for which it holds that the arc length of that path equals the geodesic offset distance d, and, for each possible path p on surface S from point q to curve C it holds that the arc length of that path equals or exceeds the geodesic offset distance d.

This problem may be relatively exotic as well as relatively `computationally expensive', so I do not expect a high implementation priority. However, maybe this request can be taken into account when selecting libraries or algorithms for similar operations.

The accuracy of the approximation improves with a decreasing sweep profile radius, provided that the surface-surface intersections are precise enough. In Onshape, the "split face" feature could be used to achieve the approximation, but there seems to be no option (at this time of writing) to keep the sweep profile normal to the sweep path.

PS: I was completely shocked (in a very positive way) when I saw OnShape for the first time in my life this week. It addresses many headaches that make CAD-life miserable. I could not believe that this CAD system strategy would work so well.
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## Comments

• Member, Moderator, Onshape Employees, Developers Posts: 2,001
This could be a good candidate for a custom FeatureScript in the future.
Lou Gallo / PD/UX - Support - Community / Onshape, Inc.
• Onshape Employees, Developers, HDM Posts: 1,175
https://cad.onshape.com/documents/669268952005185529cd2a7f/v/2ee00043a7b677763fbcc824/e/917553b66b3f68cf66442578 is a FeatureScript implementation of the approximation without spending too much time making it robust (and using some not-yet-documented FS features).  Doing the exact computation is not easy and we are unlikely to have it anytime soon.
Ilya Baran \ VP, Architecture and FeatureScript \ Onshape Inc
• Member Posts: 19 ✭✭✭
edited March 2017
@ilya_baran: Interesting, thanks for sharing.
Erratum: In the right figure, swept surface S should be swept surface R.
• Member, Onshape Employees Posts: 104
Well, after a bit of a break since the last post in this thread... it might be time to try the Offset curve released in 1.175!
• Member Posts: 19 ✭✭✭
@GregBrown: Wow, you guys did it! Amazing, thank you for taking the time to update this ancient thread!
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