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  Page
Primitive Type type
 ⊞  For information on the different types, see the Primitive and Spline articles. Depending on the primitive type chosen, some SOP options may not apply. Using the 'Primitive' primitive type is not recommended when using instancing.
 Primitive
prim

 Polygon
poly

 NURBS Curve
nurbs

 Bezier Curve
bezier

Orientation orient
 ⊞  The plane on which the circle lies.
 XY Plane
xy

 YZ Plane
yz

 ZX Plane
zx

Modify Bounds modifybounds
 Available only when an input is connected to the Curcle SOP to set bounds for the sphere. When Modify Bounds = On the transform parameters below will further modify the radius and position of the bounds.
Radius rad
 ⊞  The Radius of the Circle in the X and Y directions.
 X
radx

 Y
rady

Center t
 ⊞  The Center of the Circle in X, Y and Z.
 X
tx

 Y
ty

 Z
tz

Reverse Anchors reverseanchors
 Invert the direction of anchors.
Anchor U anchoru
 Set the point in X about which the geometry is positioned, scaled and rotated.
Anchor V anchorv
 Set the point in Y about which the geometry is positioned, scaled and rotated.
Order order
 If a spline curve is selected, it is built at this order.
Divisions divs
 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 arc
 ⊞  Determines how the circle should be drawn. Applies to polygons and imperfect splines only.
 Closed
closed
 An enclosed curve.
 Open Arc
openarc
 An open curve segment.
 Closed Arc
closedarc
 An Open Arc with connecting edges to the centre resembling a slice of pie.
 Sliced Arc
slicedarc
 Same as Closed Arc, but connects every single point to the center of the circle.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 angle
 ⊞  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.
beginangle

endangle

Imperfect 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.
Texture Coordinates texture
 ⊞  Option to include texture cooordinates or not.
 Off
off
 No texture coordinates are created.
 Face
face
 Default texture coordinates are created. To modify texture coordinates, see Texture SOP.
Compute Normals normals
 When On, normals are created for the surface.
Operator Inputs
 Input 0: 
Info CHOP Channels
Extra Information for the Circle SOP can be accessed via an Info CHOP.
Common SOP Info Channels
 num_points  Number of points in this SOP.
 num_prims  Number of primitives in this SOP.
 num_particles  Number of particles in this SOP.
 last_vbo_update_time  Time spent in another thread updating geometry data on the GPU from the SOP's CPU data. As it is part of another thread, this time is not part of the usual frame time.
 last_meta_vbo_update_time  Time spent in another thread updating meta surface geometry data (such as metaballs or nurbs) on the GPU from the SOP's CPU data. As it is part of another thread, this time is not part of the usual frame time.
Common Operator Info Channels
 total_cooks  Number of times the operator has cooked since the process started.
 cook_time  Duration of the last cook in milliseconds.
 cook_frame  Frame number when this operator was last cooked relative to the component timeline.
 cook_abs_frame  Frame number when this operator was last cooked relative to the absolute time.
 cook_start_time  Time in milliseconds at which the operator started cooking in the frame it was cooked.
 cook_end_time  Time in milliseconds at which the operator finished cooking in the frame it was cooked.
 cooked_this_frame  1 if operator was cooked this frame.
 warnings  Number of warnings in this operator if any.
 errors  Number of errors in this operator if any.
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