Why is the most upper and lower points on this sphere sharp?

I tried to rebuild the following sphere to degree 3. When I move the control points I those two points turns into sharp points. How can I fix this?Untitled.3dm (281.7 KB)

Because those are the singularities (zero length edges), and you moved them.

A NURBS sphere starts as a rectangle that is rolled into a tube.
The left and right edges of the untrimmed surface are joined into a “Seam”.
The round ends of the tube are shortened to a zero length making them singularities.

What problem are you trying to solve?

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I want to get a smooth result when I manipulate any control point. as you see those two points are not smooth, they are sharp. How is it possible?

I would play around with CageEdit.


To keep things smooth, move the singularity and the next ring of points together - if those stay in plane with each other, the smoothness will be maintained.