Circle
surface node
Creates open or closed arcs, circles and ellipses.
This Operation is used to create circles and ellipses. If you click and drag the mouse, it generates a circle whose radii are specified by your drag.
Clicking the mouse button on the Construction Plane without dragging places a circle with radii specified in the Parameters dialog box (default of 1) at the location of the mouse click. The radii of the default circle are aligned with the Construction Plane’s X and Y axis.
Typing Enter places a circle or ellipse whose size and position are specified in the Parameters dialog. The radii of the default circle are aligned with the Construction Plane’s X and Y axis.
If an odd aspect ratio was previously entered in the Parameters dialog, clicking and dragging produces circles which maintain that aspect ratio. This can be reset by clicking on the Reset Radii button.
If two NURBS circles that are non-rational (i.e. their X and Y radii are unequal) are skinned, more ioparms may be generated than expected. This is because non-rational NURBS circles parameterize 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 SOP or Transform SOP to deform the second copy before skinning.
Placing a Circle in the viewer
| To... | Do this |
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Place the circle anywhere in the scene |
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Place the circle at the origin |
The circle can be moved once it is placed by either dragging it in the scene view or changing the values in the parameter editor.
Parameters
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Primitive Type |
Type of geometry created. |
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Orientation |
Orientation of the circle. |
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Radius |
These are the X and Y radii of the circle. Entering non-equal values in the xy fields results in elliptical shapes. |
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Center |
Location of the center of the circle. |
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Order |
Sets the spline order when building a circle with a Bezier or NURBS curve type. The lowest order is 2 (linear); the highest is 11. Cubic curves are built by default. |
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Divisions |
The number of points + 1 used to describe the circle. This option applies to polygons and imperfect NURBS only. 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. For open arc types, the number of points will equal Divisions + 1, and for closed arc types, Divisions + 2. The number of points on a Bezier circle will be higher than the number of divisions specified, based on the order of the Bezier curve. The # of Divisions is ignored when building a perfect (rational) NURBS or Bezier circle. Tip
Set the Divisions to 3 to create Triangles. |
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Arc Type |
This menu provides you with the choices: Closed, Open Arc, Closed Arc, and Sliced Arc. The difference between these is illustrated below:
This option is disabled when building a perfect (rational) NURBS or Bezier circle. To remove a part of the rational curve later, you can use the Carve SOP. The Closed and Closed Arc options are primarily meant for polygonal circles. |
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Arc Angles |
When making an arc rather than a full circle, these values specify the starting and ending points of the arc in degrees. This option is disabled when building a perfect (rational) NURBS or Bezier circle. |
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Imperfect |
Specifies whether the NURBS / Bezier circle should be built using rational or non-rational splines. A perfect circle has a rational topology: one that associates non-unit weights with certain vertices. Furthermore, a perfect circle has a predefined number and positions of CVs for any given spline order. An imperfect circle is non-rational and its number of CVs isn’t that strictly determined by its order. Rational circles built this way yield a mathematically perfect shape; however, given their special definition, perfect circles are not always the ideal choice for further modeling of their points. Besides, they represent heavier geometry and may put more pressure both on the cpu and ram. In practice, you will find imperfect circles to be a better modeling choice, so it is advisable to build perfect circles only when perfect shapes are paramount. |
Example files
Circle
$HFS/houdini/help/examples/nodes/sop/circle/Circle.cmd
This is an example of the different geometry types and arc types a circle can have.
Geometry types include primitives, polygons, NURBS, and Beziers.
Arc types include closed circle, open arc, closed arc, and sliced arc.
The arc examples are animated, so playback the animation to see the arcs opening.