# RhinoCommon - working with arcs/circles

Hi all,

Been trying to work with arcs and circle objects this morning, and I’m having some trouble understanding how to do a few things… I would like to construct an arc from the following available data: center point, radius, start/end points (which I know are on the arc)… Obviously, there are two possible arcs here, that is not an issue at the moment, I can take care of that later.

I haven’t figured out how to construct the arc (Arc or ArcCurve) directly with this data - although I’m sure it’s possible…

The other direction I thought to try was to simply construct a circle with center and radius and split it at the two points. But Circle objects do not have a Split() method… I can certainly convert the circle object ToNurbsCurve() and split that at the points, but somehow there has to be a better, more “analytical” way…?

Anyway, for what I was doing, I worked around it differently, but I am now curious as to how to do this correctly…

Thx, --Mitch

Hi Mitch,

Think of arcs as oriented planes with a radius and angle. The origin of the arc is always a point on the plane’s x-axis. The angle is measured from the x-axis counter-clockwise (as you’d expect). The radius should be obvious.

To create an arc from a point and radius, create a plane (world x-y will do). Set the origin of the plane to the point. Then use `rs.AddArc`.

For the second arc, you can use `rs.AddArc3Pt`, correct?

The RhinoCommon basis for all this is a `Rhino.Geometry.Arc` object. If you look at the online help, you will find lots of other construction methods.

An `Arc` is just a simple structure. The curve representation of an arc is a `Rhino.Geometry.ArcCurve` object. You can create one of these by passing a `Rhino.Geometry.Arc` object. Again, reference the help if you need. Using an `ArcCurve`, there will be no reason to `ToNurbsCurve` the arc…

Hi Mitch
v1 = Vector( center_pt -> start_pt )
v2 = Vector( center_pt -> end_pt )
v3 = Unitize( v1 + v2 )
radius = Distance( center_pt, start_pt )
mid_pt = center_pt + v3 * radius
arc = Arc_3_Points( start_pt, mid_pt, end_pt )
… Assuming the arc is not 180 degrees …
( just an idea … I have not tried that … )
Cheers - emilio

Hi Emilio,

Thanks, that’s a great way to get the angle bisector for the third arc point… Will try to remember that!

Cheers,
–Mitch

Thanks for the info Dale… Finally this is actually the simplest way to go:

``````radius=somevalue
arcPlane=Rhino.Geometry.Plane(arcCtr,endPts[0],endPts[1])
angle=Rhino.Geometry.Vector3d.VectorAngle(endPts[0]-arcCtr,endPts[1]-arcCtr)