Circle SOP
Summary
The Circle SOP creates open or closed arcs, circles and ellipses.
If two NURBS circles that are nonrational (i.e. their X and Y radii are unequal) are skinned, more isoparms may be generated than expected. This is because nonrational NURBS circles parameterise their knots based on chord length, and the Skin SOP must consolidate the total number of knots between the two circles before skinning.
To remedy this, you may want to use a Refine SOP, and unrefine the resulting skin, or better yet  before unrefining, start with the same circle and use a Primitive or Transform SOP to deform the second copy before skinning.
Parameters
Primitive Type  For information on the different types, see the Geometry Types Guide. Depending on the primitive type chosen, some SOP options may not apply.
 Primitive
 Polygon
 NURBS Curve
 Bezier Curve
Orientation  The plane on which the circle lies.
Radius / center /radx /rady /tx /ty /tz
 The Radius of the Circle in the X and Y directions, and the Center of the Circle in X, Y and Z.
Order  If a spline curve is selected, it is built at this order.
Divisions /div
s  The number of edges (points +1) used to describe the circle. This option applies to polygons and imperfect splines. The more Divisions a circle has, the smoother it looks. Using three divisions makes a triangle, four divisions a diamond, five divisions a pentagon, and so on. Also, for open arc types, the number of points will equal Divisions + 1, and for closed arc types, Divisions + 2.
Arc Type  Determines how the circle should be drawn. Applies to polygons and imperfect splines only.
 Closed  An enclosed curve.
 Open Arc  An open curve segment.
 Closed Arc  An Open Arc with connecting edges to the centre resembling a slice of pie.
 Sliced Arc  Same as Closed Arc, but connects every single point to the center of the circle.
Arc options are available for polygonal circles and some spline types. The difference between the choices: Open, Closed, and Slice are illustrated below for the polygonal case:
Arc Angles  The beginning and ending angles of the arc. An arc will start at the beginning angle, and proceed towards the ending angle. If beginning=0 and end=360 it will be a full circle. As a reference:
Note: The total angle can exceed 360, making multiple wraps of the circle.
Imperfect  This option applies only to Bezier and NURBS circles. If selected, the circles are approximated nonrational curves, otherwise they are perfect rational closed curves.
Inputs / Geometry Types
Primitive, Polygon, Bezier, NURBS curves.
