tangent of surface along U or V

adamohern · May 2016

I can create a plane that is tangent to a surface at a given point.

1) How can I force the plane to align its X axis to the U or V direction of the surface at the intersection point?

2) How can I more generally extract the U or V tangent direction of a surface at a point?

kevin_o_toole_1 · May 2016

We don't have something for that exposed (though of course you could approximate it with multiple calls to evTangentPlane).

I'm curious: What do you want UV directions for? In general, neither the end user nor the FS user has control over the internal parameterization of a given surface, so writing code that depends on the parameterization could behave differently than you or the end user expects.

adamohern · May 2016

Mainly for experiments with generative design tools in Onshape. With generative designs it can be fun to flow repetitive or evolving patterns across the contours of a surface, and in some cases you'll want that to flow right along the U or V direction of a surface. Google image search "generative architecture" for examples.

lougallo · June 2016

@adamohern Try this https://cad.onshape.com/documents/68455a5d522e81cd65a35211/w/deb148ff386c8a32e35e7d65/e/929b7ca8aeee162bb0f129d9

adamohern · June 2016

Thanks Lou, but I'd already gotten this far. My question is about defining the X-direction of the plane to be parallel to either the U or V of the tangent surface. As it is, the x-direction is simply canonical, with no option for alignment with the flow of the surface.

billy2 · June 2016

Thanks for sharing solution Lou, I'm currently working on this. Your T Plane is nicely documented.

So I have a skPoint (a point in a sketch) and project this to the blue surface then create a plane. This is Lou's cool solution.

Things of interest:
-When querying a skPoint, world coordinates are returned. This is good because I don't have to transform it to know where it is in world space. The bad is that it doesn't know about how it was created ie.. what's the direction of the plane it was created on.

-The projection is a minimum distance from the point to the surface vs. a normal projection from sketch to surface.

If you are trying to create a 3D point on geometry using this technique, one would expect a normal projection based on the sketch view. The minimal distance isn't intuitive.

Using a sketch to create a 3D point on a surface will be off because evDistance returns minimal distance and not normal distance.

Gray line is the minimal distance and the orange line is the normal projection from sketch which is what I want.

Using a 2D sketch to generate a 3D point with a minimal distance projection isn't going to be intuitive and therefore not really a good way to create a 3D point on geometry. I'm trying to figure out how to generate a normal projection.

Is there an ev..... function that'll give you the intersection between a vector and a face?

Thanks for your help Lou,

dave_cowden · July 2016

Hi, I created a feature that does ( I think ) exactly what you are doing. It provides ability to create a plane tangent to a surface and aligned with a direction. It ends up being pretty tricky for aribrary surfaces, but with some math its not to bad for built- in surfaces:

https://cad.onshape.com/documents/0de04acb5bac92ca16796edf/w/cdd1d1882311084fe54bff24/e/daaec91732ca5add243c90dd

See "Create tangent plane aligned with another vector"

adamohern · July 2016

Thanks Dave. I knew how to do that as well. The problem is getting the actual U and V tangent directions for an input surface. That's what this thread was meant to be about from the beginning. Creating tangent planes on a surface is trivial. It's aligning them to the flow of an arbitrary surface that's impractical.

tangent of surface along U or V

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