Problem surface... Won't intersect or trim. Seems okay though? I can't find anything weird

I have a problem surface here maybe someone can help me with.

Both top and bottom surfaces here are identical, save for arbitrarily moving one point in XZ (circled in red), which shouldn’t have any effect you would think? Top example shows that it intersects and trims correctly with the surface on xz plane. Bottom is the untouched surface that won’t correctly intersect and trim.

I have trimmed most of the surface away, just to protect client’s data but that doesn’t seem to have affected the behavior.

problem surface.3dm (158.5 KB)

That one point is a corner point on the surface - it is placed so that at that location the U and V of the surface are very nearly tangent. My guess is this is getting in the way of the surface-surface intersection. If you use the edge curves, the trim is fine. In general it’s best not to have adjacent edges of a surface be tangent to one another - a lot of operations depend upon being able to calculate a surface normal and that is underfined when U and V are in line. So, offsetting (and filleting) may not work, as well as, apparently anyway, surface to surface intersections.


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Understood. Thanks Pascal.

Yeah, I mean it’s a boat hull so I sortof depend on this strategy to make a single surface hull with a round forefoot.

End goal is a trim in Grasshopper, so if I grab the edges should be good to go then. Normally I’d just do that if it gave me grief in Rhino but it’s kindof a bunch of extra work in Grasshopper.

Alternatives to avoid u and v directions becoming tangent: Boat hulls with smoothly curve stems - how to model

Thanks David.

I’ve always shied away from the trimmed surface option because it becomes such a nuisance with orca having to redo the trim every time you want to check the hydros and it also breaks (or at least it did last time I tried but years ago) the real-time hydros in Orca. I might revisit this now that I’m dabbling in grasshopper.

I find it too hard to fair the topsides where it really matters with the 3-sided option.

Which leaves us with the tangent UVs. At least the ugly part is usually underwater this way.