Sweep1 question

After following another thread, I was plying with Sweep1 and noticed something that I couldn’t understand. In the attached images you can see that the surface is broken up at places that don’t obviously correspond to features on either of the profile curves, (a circle and a triangle). I would expect the continuity of the surface to break at the points of the triangle and gradually diminish towards the circle but there is an additional break for each side of the triangle.
What causes this?

Hi Nick- that should work out better if you align the seam point for Sweep1 at a corner (End Onsap) of the square.


@Pascal, it isn’t a square, it’s a triangle, and the seam point is at a corner. Here is the file.sweep1.3dm (208.7 KB)

The transition looks better if you rebuild the circle with 10 or 12 points first and align the top of the triangle with the quad point on the circle.
Transitioning from a circle with 4 fully multiple knots to a triangle adds some creases that you don’t want.

Thanks Lowell. Is this because a Rhino circle is a special object. I seem to remember seeing comments that it was. I have no idea what “4 fully multiple knots” means, but is that a situation that I wouldn’t expect to see with an “ordinary” closed curve?

Ya, its kind of special.
When you make a circle, it’s defined by a plane and a radius as you would expect.
When you use it to make a nurbs surface, it gets converted to a nurbs curve first and the most accurate way to represent a circle with a nurbs curve is with 4 rational nurbs arcs stuck together. There is a fully multiple knot at the joint between arcs. That’s a nurbs structure that allows a kink in the middle of a curve. (I should have left that part out) You can kind of see the implications of that if you turn on the points on a circle and drag some of them around.
Rebuild makes the circle into a close approximation of a circle that doesn’t have the same 4-arc structure. The reason we don’t automatically convert to that all the time is that its not an exact circle shape, just a close fit to a circle.

Thanks for the explanation. So when using circles to make any surfaces, its generally better to rebuild them?

I don’t think I’d go that far.
It’s really more when there is some kind of transition from a circle to some other shape.
Things like lofting between different size circles or extruding will generally make cleaner surfaces when you don’t rebuild.

Got it. thanks.