Acad Polylines

AutoCAD Polylines

Use this skill when creating, reading, or modifying polylines — lightweight polylines (Polyline / LWPOLYLINE), 2D polylines (Polyline2d), 3D polylines (Polyline3d), vertex manipulation, bulge values, and curve extraction.

Polyline (Lightweight / LWPOLYLINE)

Creating a Polyline

Polyline polyline = new Polyline();
polyline.Layer = layerName;

for (int i = 0; i < points.Count; i++)
{
    Point2d pt = points[i];
    double bulge = i < bulges.Count ? bulges[i] : 0.0;
    polyline.AddVertexAt(i, pt, bulge, 0.0, 0.0);
    // AddVertexAt(index, point, bulge, startWidth, endWidth)
}

polyline.ColorIndex = 256;  // ByLayer
// or: polyline.Color = Color.FromRgb(255, 0, 255);

BlockTableRecord ms = (BlockTableRecord)tr.GetObject(
    bt[BlockTableRecord.ModelSpace], OpenMode.ForWrite);
ms.AppendEntity(polyline);
tr.AddNewlyCreatedDBObject(polyline, true);

Reading Vertices

Polyline pline = tr.GetObject(entityId, OpenMode.ForRead) as Polyline;

int count = pline.NumberOfVertices;
for (int i = 0; i < count; i++)
{
    Point2d pt = pline.GetPoint2dAt(i);      // 2D vertex in OCS
    Point3d pt3d = pline.GetPoint3dAt(i);    // 3D vertex (applies Elevation)
    double bulge = pline.GetBulgeAt(i);      // arc curvature to next vertex
    double startW = pline.GetStartWidthAt(i);
    double endW = pline.GetEndWidthAt(i);
}

Modifying Vertices

pline.UpgradeOpen();  // or open ForWrite initially

// Move a vertex
pline.SetPointAt(2, new Point2d(10.0, 20.0));

// Change arc curvature
pline.SetBulgeAt(2, 0.5);

// Change width at a vertex
pline.SetStartWidthAt(2, 0.0);
pline.SetEndWidthAt(2, 1.0);

// Insert a new vertex at index 3
pline.AddVertexAt(3, new Point2d(15.0, 25.0), 0.0, 0.0, 0.0);

// Remove vertex at index 3
pline.RemoveVertexAt(3);

Bulge Values

The bulge value at vertex i defines the arc from vertex i to vertex i+1:

bulge = tan(includedAngle / 4)

Calculating Bulge

// From included angle (radians)
double bulge = Math.Tan(includedAngle / 4.0);

// Back to angle
double angle = 4.0 * Math.Atan(Math.Abs(bulge));  // always positive

Polyline as Curve

Polyline inherits from Curve. All Curve methods are available.

Curve Methods

// Point at a given distance along the curve
Point3d pt = pline.GetPointAtDist(50.0);

// Distance to a point on the curve
double dist = pline.GetDistAtPoint(pt);

// Closest point on curve to an arbitrary point
Point3d closest = pline.GetClosestPointTo(testPoint, false);
// second param: extend curve if needed

// Tangent direction at a point
Vector3d tangent = pline.GetFirstDerivative(pline.GetParameterAtPoint(pt));

Offset and Split

// Offset polyline by distance (positive = left, negative = right relative to direction)
DBObjectCollection offsets = pline.GetOffsetCurves(2.5);
foreach (DBObject obj in offsets)
{
    Entity offsetEnt = (Entity)obj;
    ms.AppendEntity(offsetEnt);
    tr.AddNewlyCreatedDBObject(offsetEnt, true);
}

// Split at a point
DBObjectCollection pieces = pline.GetSplitCurves(
    new Point3dCollection { splitPoint });

// Reverse direction
pline.ReverseCurve();

Segment-Level Access

for (int i = 0; i < pline.NumberOfVertices - 1; i++)
{
    SegmentType segType = pline.GetSegmentType(i);

    if (segType == SegmentType.Line)
    {
        LineSegment2d lineSeg = pline.GetLineSegment2dAt(i);
        double length = lineSeg.Length;
    }
    else if (segType == SegmentType.Arc)
    {
        CircularArc2d arcSeg = pline.GetArcSegment2dAt(i);
        double radius = arcSeg.Radius;
        Point2d center = arcSeg.Center;
    }
}

SegmentType Enum

• SegmentType.Line — straight segment
• SegmentType.Arc — arc segment
• SegmentType.Coincident — zero-length segment (coincident vertices)
• SegmentType.Point — degenerate segment
• SegmentType.Empty — no segment

Polyline2d (Heavy / DXF "POLYLINE")

Polyline2d vertices are separate database objects (Vertex2d) owned by the polyline.

Creating Polyline2d

Point3dCollection pts = new Point3dCollection
{
    new Point3d(0, 0, 0),
    new Point3d(10, 0, 0),
    new Point3d(10, 10, 0)
};

Polyline2d pline2d = new Polyline2d(Poly2dType.SimplePoly, pts, 0.0, false, 0.0, 0.0, null);
// (type, points, elevation, closed, defaultStartWidth, defaultEndWidth, bulges)

ms.AppendEntity(pline2d);
tr.AddNewlyCreatedDBObject(pline2d, true);

Iterating Polyline2d Vertices

Polyline2d pline2d = tr.GetObject(entityId, OpenMode.ForRead) as Polyline2d;

foreach (ObjectId vertexId in pline2d)
{
    Vertex2d vertex = tr.GetObject(vertexId, OpenMode.ForRead) as Vertex2d;
    if (vertex == null) continue;

    Point3d position = vertex.Position;
    double bulge = vertex.Bulge;
    double startWidth = vertex.StartWidth;
    double endWidth = vertex.EndWidth;
}

Polyline3d

Polyline3d vertices are separate database objects (PolylineVertex3d) with full 3D coordinates.

Creating Polyline3d

Point3dCollection pts = new Point3dCollection
{
    new Point3d(0, 0, 0),
    new Point3d(10, 0, 5),
    new Point3d(20, 10, 10)
};

Polyline3d pline3d = new Polyline3d(Poly3dType.SimplePoly, pts, false);
ms.AppendEntity(pline3d);
tr.AddNewlyCreatedDBObject(pline3d, true);

Iterating Polyline3d Vertices

Polyline3d pline3d = tr.GetObject(entityId, OpenMode.ForRead) as Polyline3d;

foreach (ObjectId vertexId in pline3d)
{
    PolylineVertex3d vertex = tr.GetObject(vertexId, OpenMode.ForRead) as PolylineVertex3d;
    if (vertex == null) continue;

    Point3d position = vertex.Position;
}

Handling All Polyline Types

When processing an entity that could be any polyline type, use a cascade pattern:

private static List<Point3d> GetVertices(Entity entity, Transaction tr)
{
    List<Point3d> points = new List<Point3d>();

    Polyline pline = entity as Polyline;
    if (pline != null)
    {
        for (int i = 0; i < pline.NumberOfVertices; i++)
            points.Add(pline.GetPoint3dAt(i));
    }
    else
    {
        Polyline2d pline2d = entity as Polyline2d;
        if (pline2d != null)
        {
            foreach (ObjectId vId in pline2d)
            {
                Vertex2d v = tr.GetObject(vId, OpenMode.ForRead) as Vertex2d;
                if (v != null) points.Add(v.Position);
            }
        }
        else
        {
            Polyline3d pline3d = entity as Polyline3d;
            if (pline3d != null)
            {
                foreach (ObjectId vId in pline3d)
                {
                    PolylineVertex3d v = tr.GetObject(vId, OpenMode.ForRead) as PolylineVertex3d;
                    if (v != null) points.Add(v.Position);
                }
            }
        }
    }

    return points;
}

Common Recipes

Join Polylines

// Join contiguous polylines into one (endpoints must be within tolerance)
pline.UpgradeOpen();
pline.JoinEntities(new Entity[] { otherPline1, otherPline2 });

Reverse Direction

pline.UpgradeOpen();
pline.ReverseCurve();

Convert Polyline to Point Collection

Point3dCollection points = new Point3dCollection();
for (int i = 0; i < pline.NumberOfVertices; i++)
    points.Add(pline.GetPoint3dAt(i));

Spline Fit a Polyline2d

pline2d.UpgradeOpen();
pline2d.ConvertToPolyType(Poly2dType.CubicSplinePoly);
// Also: QuadSplinePoly, FitCurvePoly

Gotchas

• Polyline (LWPOLYLINE) stores 2D vertices only — Z comes from the Elevation property
• Polyline vertices are in OCS (object coordinate system), not WCS — convert with Matrix3d.WorldToPlane(Normal) if the normal is not ZAxis
• AddVertexAt index must be 0 to NumberOfVertices inclusive — index = NumberOfVertices appends
• RemoveVertexAt on a polyline with only 2 vertices throws — erase the entity instead
• Polyline2d and Polyline3d vertices are separate database objects — must open each via transaction
• DXF name "LWPOLYLINE" = Polyline class; "POLYLINE" = Polyline2d or Polyline3d — check the managed type to distinguish
• GetOffsetCurves returns DBObjectCollection — results may not be Polyline (could be individual curves for complex offsets)
• Area only works on closed polylines in a plane — throws for non-planar 3D polylines
• Bulge at vertex i defines the arc from vertex i to vertex i+1 (not centered on vertex i)
• Closed = true adds a segment from last to first vertex — does not duplicate the first point
• JoinEntities requires contiguous endpoints within Application.DocumentManager.MdiActiveDocument.Database.EqualPoint tolerance
• Polyline2d foreach yields ObjectIds of all sub-entities including fit/spline vertices — check VertexType to filter
• ConvertToPolyType on Polyline2d is destructive — original vertices become control/fit points

Related Skills

• acad-geometry — Point2d/Point3d construction, bulge math, CircularArc2d from arc segments
• acad-editor-input — selecting polylines with GetEntity or selection filters (DxfCode.Start "LWPOLYLINE")
• acad-layers — setting polyline layer, reading layer properties
• acad-hatches — polylines as hatch boundary loops (AppendLoop with entity IDs or vertex+bulge arrays)