RhinoCommon - working with arcs/circles

Scripting
1

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

2

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…

3

Hi Mitch
What about this way … (in pseudo-code) ?
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

4

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

5

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)
arc=Rhino.Geometry.Arc(arcPlane,radius,angle)

Cheers,

–Mitch

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