Hi Jean-Paul,

RE: two curves

> The bottom one is the shape I want (based on a blueprint, etc.).
> I built it by blending two straight lines with a G2 blend,
> thus getting a 3-segment curve. If revolved, lofted etc.,
> it will show identical segments in the surface, a less
> than desirable outcome if it can be avoided

I have to disagree with your statement, the curve you created with the G2 blend will give you a technically smoother surface than the top 'single segment' curve.
The joins between those curves have no bearing on the smoothness, they are part of the NURBS world, if you were to create a surface, either by revolving, lofted etc. from the G2 curve and rendered it, there would be no signs of those joins, it would show a continuous smooth surface, and the 'single segment' curve would look good also, but it's not perfect.

I try to model with the simplest geometry possible, lines, circles and arcs where possible, in that way I know I have sound geometry, when I use the modeling tools on these curves things tend to work more smoothly, like Booleans and blends.

I reproduced your curves and took them into a CAD program that has analysis tools, (I think Rhino has the same tools) to show you what I mean.

Here are the curves in MoI, I added an additional one which is made up of 5 segments, 3 straight lines and 2 arcs.



Here are the three curves analysed showing there curvature combs, the smoother the comb the better.



As you can see the top curve is the most undesirable, technically, If you were asked to model something by a company who required A-class surfaces for that shape, sorry to say but the top 'single segment' curve would be rejected, if they analysed it that is.

If Rhino has the comb tool you can manipulate the 'single segment' curve until you get smooth looking combs.

Cheers