Sweep Lumpiness?

S1mon

I'm trying out the latest updates - specifically Bridging Curve - and I did some sweeps to see the results as a surface. The results are disappointing.

The first image shows the curvature plots of the two bridging curves (degree-5 Béziers) which were used to generate the surface. By definition these are infinitely differentiable within themselves and therefore C^∞. The second shows the curvature of the surface. The V (blue) direction curvature plots aren't too bad, but you can see some slight discontinuities in the curvature which implies they are not C3. In the U (red) direction, the issues are much more pronounced as it sweeps around the corner. The curvature oscillates in a way which indicates that the surface is likely a degree-3 B-spline approximation, with a lot of spans and approximation issues. Perhaps sweeps should not be used to create Class-A surfaces?

Perhaps this is a Parasolid issue, as I often found Solidworks to have similar crappy results using sweep.

ilya_baran

Think about one of the isocurves that has the lumpy comb -- it's at a constant offset relative to the sweep path.  Even if the sweep path is a B-spline, that offset curve is very likely to be not representable exactly as a B-spline, so Parasolid approximates, and it uses cubic B-splines when it does that (which causes the "lumpiness").  This is G2 and totally fine for anything other than class-A surfacing (which at this point in time Onshape doesn't have great support for anyway).  There's definitely room for improvement, but even if that surface was made of polished metal, I doubt you'd be able to see the irregularities with your naked eye -- the discontinuities are pretty small.

S1mon

@ilya_baran

I figured that’s what’s happening. It would be nice if there was an option to use approximate offsets. Rhino and Alias have options to rebuild the curves for sweeps and other surfacing operations. 

I agree that the surface itself is more than clean enough for most things. The problem with non-class-A surfaces is when you start building other things off of edges or trims. It ends up being like a photocopy of a photocopy of a photocopy… Distortions get added, and some operations don’t work so well because there are now much “heavier” surfaces and curves.