Why does one loft create a polysrf and the other a surface

[miano]

More things that have been keeping me up at night. In this file I have two red arc and two green cards. If I loft the red arcs I get a surface. If I loft the green args I get a polysrf. Why the difference?

Problem Loft 2.3dm.zip (14.6 KB)

[Lagom]

The left red curve is a NURBS arc, and the right red curve is a “true” arc (radius and angle recognised). You’ll also see their curvature graphs visually differ. The left and right green curves are both “true” arcs, and so you get two surfaces joined into a polysurface.

[miano]

How does one create a Nurbs arc v. an arc?

[Lagom]

I have no idea how you created your curves. I never use arcs in my field of work. I only do traditional NURBS surface modelling and must avoid curves with kinks. Maybe this is useful:

@davidcockey

A user can transform an exact arc which is a rational degree 2 NURBS curve into an exact rational degree 3 NURBS curve using ChangeDegree with Deformable=No. The degree 3 form of an arc will have multi-knots even though curvature is continuous.

@jeremy5

Rhino identifies a degree 3 arc as a “Nurbs Arc” as distinct from an Arc

@pascal

NURBS arcs in Rhino are indeed made as degree 2 but what makes them shaped exactly is the control point weighting. You can ChangeDegree to 3 but these curves will still be rational.

[miano]

I either use the arc or circle commands from the menu. I am puzzled at the moment why some are NURBS arc and others are just arcs.