Curve.md

Curve Class

Overview

The Curve class is the base class for all curve-based entities in AutoCAD including lines, arcs, circles, polylines, ellipses, and splines. It provides common curve properties and methods.

Namespace

Autodesk.AutoCAD.DatabaseServices

Inheritance Hierarchy

System.Object
  └─ RXObject
      └─ DBObject
          └─ Entity
              └─ Curve
                  ├─ Line
                  ├─ Arc
                  ├─ Circle
                  ├─ Ellipse
                  ├─ Spline
                  ├─ Polyline
                  ├─ Polyline2d
                  └─ Polyline3d

Key Properties

Property	Type	Description
StartPoint	Point3d	Gets the start point of the curve
EndPoint	Point3d	Gets the end point of the curve
StartParam	double	Gets the start parameter value
EndParam	double	Gets the end parameter value
Closed	bool	Gets whether the curve is closed
Periodic	bool	Gets whether the curve is periodic

Key Methods

Method	Return Type	Description
GetPointAtParameter(double)	Point3d	Gets point at parameter value
GetParameterAtPoint(Point3d)	double	Gets parameter at point
GetDistAtParam(double)	double	Gets distance along curve at parameter
GetParamAtDist(double)	double	Gets parameter at distance
GetPointAtDist(double)	Point3d	Gets point at distance along curve
GetDistAtPoint(Point3d)	double	Gets distance along curve to point
GetClosestPointTo(Point3d, bool)	Point3d	Gets closest point on curve
GetFirstDerivative(double)	Vector3d	Gets tangent vector at parameter
GetSecondDerivative(double)	Vector3d	Gets curvature vector at parameter
GetSplitCurves(Point3dCollection)	DBObjectCollection	Splits curve at points
Extend(double)	void	Extends curve to parameter
Extend(bool, Point3d)	void	Extends curve to point

Code Examples

Example 1: Getting Points Along a Curve

using (Transaction tr = db.TransactionManager.StartTransaction())
{
    Curve curve = tr.GetObject(curveId, OpenMode.ForRead) as Curve;
    
    // Get points at regular intervals
    double totalLength = curve.GetDistAtParam(curve.EndParam);
    int numPoints = 10;
    
    for (int i = 0; i <= numPoints; i++)
    {
        double distance = (totalLength / numPoints) * i;
        Point3d point = curve.GetPointAtDist(distance);
        
        ed.WriteMessage($"\nPoint {i}: ({point.X:F2}, {point.Y:F2})");
    }
    
    tr.Commit();
}

Example 2: Finding Closest Point on Curve

using (Transaction tr = db.TransactionManager.StartTransaction())
{
    Curve curve = tr.GetObject(curveId, OpenMode.ForRead) as Curve;
    
    Point3d testPoint = new Point3d(100, 100, 0);
    
    // Get closest point (extend = false means don't extend beyond endpoints)
    Point3d closestPoint = curve.GetClosestPointTo(testPoint, false);
    
    double distance = testPoint.DistanceTo(closestPoint);
    
    ed.WriteMessage($"\nClosest point: ({closestPoint.X:F2}, {closestPoint.Y:F2})");
    ed.WriteMessage($"\nDistance: {distance:F2}");
    
    tr.Commit();
}

Example 3: Getting Tangent Direction

using (Transaction tr = db.TransactionManager.StartTransaction())
{
    Curve curve = tr.GetObject(curveId, OpenMode.ForRead) as Curve;
    
    // Get tangent at midpoint
    double midParam = (curve.StartParam + curve.EndParam) / 2;
    Vector3d tangent = curve.GetFirstDerivative(midParam);
    
    // Normalize to unit vector
    tangent = tangent.GetNormal();
    
    double angle = Math.Atan2(tangent.Y, tangent.X) * (180.0 / Math.PI);
    
    ed.WriteMessage($"\nTangent angle at midpoint: {angle:F2}°");
    
    tr.Commit();
}

Example 4: Splitting a Curve

using (Transaction tr = db.TransactionManager.StartTransaction())
{
    Curve curve = tr.GetObject(curveId, OpenMode.ForRead) as Curve;
    BlockTableRecord btr = tr.GetObject(db.CurrentSpaceId, OpenMode.ForWrite) as BlockTableRecord;
    
    // Create split points
    Point3dCollection splitPoints = new Point3dCollection();
    splitPoints.Add(curve.GetPointAtDist(curve.GetDistAtParam(curve.EndParam) * 0.33));
    splitPoints.Add(curve.GetPointAtDist(curve.GetDistAtParam(curve.EndParam) * 0.66));
    
    // Split the curve
    DBObjectCollection splitCurves = curve.GetSplitCurves(splitPoints);
    
    // Add split curves to database
    foreach (DBObject obj in splitCurves)
    {
        Curve splitCurve = obj as Curve;
        btr.AppendEntity(splitCurve);
        tr.AddNewlyCreatedDBObject(splitCurve, true);
    }
    
    tr.Commit();
}

Related Objects

• Entity - Base class
• Line - Derived class
• Arc - Derived class
• Circle - Derived class
• Polyline - Derived class

References

• Autodesk Official Documentation