Anyone know of a feature for replacing multiple tangent faces with a single face?

MichaelPascoe

Looking for a feature that replaces tangent faces with a single face. Similar to how loft does when creating a single face if the profile segments are tangent. Or how 3D Fit spline does when creating a spline from selected tangent edges; except the feature would do it with faces. I know it's possible with FS, just wondering if someone has already created it.

Derek_Van_Allen_BD

I feel like I experimented with this at some point and ran into issues with fit spline surfaces not playing nice with the existing edge boundaries when I attempted it.

eric_pesty

Would the "constrained surface" be able to help.
It thought there was one used by @EvanReese to apply the attractor pattern across multiple faces but I cant' seem to find it… I was "approximate" so not sure if that would work for what you are doing.

eric_pesty

@MichaelPascoe

Pull Surface is what it's called:
https://cad.onshape.com/documents/47de2b0c7db2d5832380fcd3/w/86e1200ed3cc9b96ed6f7d7f/e/e9054bb9ce23d613323dc398

Although I think his new "drape surface" might be a better option (it looks like an evolution of the pull surface):
https://cad.onshape.com/documents/4cbe0127e5dfd2e5b9d6c70e/w/ac9093e2337214ab30f41bdb/e/57177968c0e83698e6b46c9a

MichaelPascoe

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Those are legit features @eric_pesty, but those do not replicate the underlying curvature, they approximate it.

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Derek_Van_Allen_BD

I think any solution for a single surface replacement is going to necessarily be an approximation, g1 continuous inputs can have sharp discontinuities between the surface derivatives which can't be defined by a single b-spline surface mathematically, so in the case of fit spline there's an approximation step that smooths these discontinuities out.

MichaelPascoe

@Derek_Van_Allen_BD I haven't looked at the Fit spline code. So it auto approximates even if you don't have approximation checked?

eric_pesty

Yeah I know these are approximations…

However can Parasolid even handle generating a single "face" with discontinuities like this (i.e. including a perfect cylindrical and perfectly flat face)?

Doesn't the fit spline always approximate things anyway? With the "approximate" checkbox enabling a re-approximation that allows higher deviation from the base geometry.

Derek_Van_Allen_BD

Given we're talking about a kernel level function I'm a little bit reading the tea leaves here and making assumptions, but given the stuff that's exposed in the 3d fit spline feature yes it's always an approximation. The approximate checkbox is more like an "approximate harder" setting.

Interestingly the curvature analysis tool shows some real sharp discontinuities like I was describing when you use fit spline through tangent curves but still builds smooth geometry when lofted.

But by showing the control point grids I can tell that what's happening is that those curvature combs aren't actually sharp discontinuities, they're actually really insanely tight continuities where they're just going absolutely ballistic with adding control points to the whole chain to wrangle the spline into whatever internal threshold they've got for the opSplineThroughEdges function.

Look at all of those grids.

If I had to take a wild guess I'd say that they're defaulting to an approximation threshold of 1e-8 meters or maybe 1e-6 meters for deviation from the input. (Based on some other tolerancing I've seen elsewhere in related scripts)

But what this means in practice is that when doing surface replacements you're gonna have to reconcile that approximated geometry with the edge boundary faces, which probably requires those boundary faces becoming a little more spliney to make ends meet.

S1mon

It's just a multi span degree-3 curve. Internally it's G2. Often where these go from an arc to a straight line, you get really ugly results, because the input is only G1 and it has to do more of an approximation to smooth that to G2.

Derek_Van_Allen_BD

Funny enough @MichaelPascoe my earlier explorations of this were in pursuit of the polar opposite script. I need lofted surfaces to tangent connected G1 analytical geometry. Computational nightmare of a script that is turning out to be though. I don't want to settle for biarc approximation of curves, I want the minimal required number of stitching surfaces.

MichaelPascoe

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Great points.

Yeah, I don't think I'm going to take on the task, too many other projects.
Just wanted to see if it had already been done as a public feature.

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S1mon

Here's @EvanReese 's Drape Surface

https://forum.onshape.com/discussion/27913/new-custom-feature-drape-surface

I haven't tried it, but it may do what you want. Actually what is your end goal for replacing a bunch of faces with a single face, is it to create a tangent mate reference?

MichaelPascoe

Occasionally I need something like this for either tangent mate, decal, attractor pattern, the texture feature, or other features that only work on one face. It just seemed like something should exist "Merge faces" or "Approximate faces". Loft does it automatically sometimes, so I know the concept is possible. An alternative that would work for some features would be to evaluate faces similar to how you can evaluate a path in FS. The problem is that a path is pretty linear, while faces would be much more difficult to match up the u v grid; still possible though.