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ObjRefCurveParameter Method
If the reference geometry is a curve or edge with a selection point, then this gets the parameter of the selection point.

Namespace: Rhino.DocObjects
Assembly: RhinoCommon (in RhinoCommon.dll) Version: 5.1.50000.0 (5.0.20693.0)
public Curve CurveParameter(
	out double parameter


Type: SystemDouble
The parameter of the selection point.

Return Value

Type: Curve
If the selection point was on a curve or edge, then the curve/edge is returned, otherwise null.
If a curve was selected and CurveParameter is called and the SelectionMethod() is not 1 (point pick on object), the curve will be returned and parameter will be set to the start parameter of the picked curve. This can be misleading so it may be necessary to call SelectionMethod() first, before calling CurveParameter to get the desired information.
using Rhino;
using Rhino.DocObjects;
using Rhino.Commands;
using Rhino.Geometry;
using Rhino.Input;

namespace examples_cs
  public class AddRadialDimensionCommand : Rhino.Commands.Command
    public override string EnglishName
      get { return "csAddRadialDimension"; }

    protected override Result RunCommand(RhinoDoc doc, RunMode mode)
      ObjRef obj_ref;
      var rc = RhinoGet.GetOneObject("Select curve for radius dimension", 
        true, ObjectType.Curve, out obj_ref);
      if (rc != Result.Success)
        return rc;
      double curve_parameter;
      var curve = obj_ref.CurveParameter(out curve_parameter);
      if (curve == null)
        return Result.Failure;

      if (curve.IsLinear() || curve.IsPolyline())
        RhinoApp.WriteLine("Curve must be non-linear.");
        return Result.Nothing;

      // in this example just deal with planar curves
      if (!curve.IsPlanar())
        RhinoApp.WriteLine("Curve must be planar.");
        return Result.Nothing;

      var point_on_curve = curve.PointAt(curve_parameter);
      var curvature_vector = curve.CurvatureAt(curve_parameter);
      var len = curvature_vector.Length;
      if (len < RhinoMath.SqrtEpsilon)
        RhinoApp.WriteLine("Curve is almost linear and therefore has no curvature.");
        return Result.Nothing;

      var center = point_on_curve + (curvature_vector/(len*len));
      Plane plane;
      curve.TryGetPlane(out plane);
      var radial_dimension = 
        new RadialDimension(center, point_on_curve, plane.XAxis, plane.Normal, 5.0);
      return Result.Success;
from Rhino import *
from Rhino.DocObjects import *
from Rhino.Commands import *
from Rhino.Geometry import *
from Rhino.Input import *
from scriptcontext import doc

def RunCommand():
  rc, obj_ref = RhinoGet.GetOneObject("Select curve for radius dimension", 
    True, ObjectType.Curve)
  if rc != Result.Success:
    return rc
  curve, curve_parameter = obj_ref.CurveParameter()
  if curve == None:
    return Result.Failure

  if curve.IsLinear() or curve.IsPolyline():
    print "Curve must be non-linear."
    return Result.Nothing

  # in this example just deal with planar curves
  if not curve.IsPlanar():
    print "Curve must be planar."
    return Result.Nothing

  point_on_curve = curve.PointAt(curve_parameter)
  curvature_vector = curve.CurvatureAt(curve_parameter)
  len = curvature_vector.Length
  if len < RhinoMath.SqrtEpsilon:
    print "Curve is almost linear and therefore has no curvature."
    return Result.Nothing

  center = point_on_curve + (curvature_vector/(len*len))
  _, plane = curve.TryGetPlane()
  radial_dimension = \
    RadialDimension(center, point_on_curve, plane.XAxis, plane.Normal, 5.0)
  return Result.Success

if __name__=="__main__":
See Also