Sweep 2 rails greyed out continuity

I cant use the edge continuity options when doing sweep 2 rails even though this second sweep I’m trying to make is using the same rails. Anybody knows how to fix this?
Also, when I use match surfaces afterwards I still don’t get a perfect result in terms of continuity which is why I really want there greyed out options.

Hi Philip - it is not possible to provide continuity when the cross section curves are rational - like true arcs - you can get the continuity options with these input curves by using the rebuild option in Sweep2.


Hello - continuity is never offered, of course, if the rails are not surface edges; if you have an example of a non-rational cross section curves graying out the option, I would like to have that, if possible.


I think I lack some basic rhino vocabulary knowledge as I don’t understand your explanation, I’ve tried to google with no luck. Do you have an example video?

Hi Philip - curve and surface control points have an associated number that designates the weight of the point - non-rational curves and surfaces have all weights set to 1 (or, strictly speaking all the same, at any number but it is typically 1). Making exact arcs, ellipses, spheres etc in NURBS requires setting control point weights differently - for example the corner points on a circle have weight .7071 (sqrt2/2 as it happens - use the weight command to check.). At any rate when they are not all the same, the object is ‘rational’ and matching surface edges in sweeps is not possible if the cross section curves include rationals… Rebuilding always makes curves non-rational…


Another thing to pay attention to is that for continuity to work, you have to select surface edges and not stand alone curves as the sweep rails, even though they may be in the same location.

This video could be helpful for getting surface edges from your guide curves to use as sweep rails