Reading Ellipses as Polylines

[bayramakgul]

Hello,
I’m currently working with ACadSharp and encountering some difficulties interpreting angles when reading elliptical arcs. I don’t need the elliptical arc as a geometric object per se — what I actually need is to convert it into a polyline (i.e., a series of approximated points or segments).

Is there a recommended way to extract or approximate elliptical arcs as polylines using ACadSharp?
Any guidance or code examples would be greatly appreciated.

Thank you in advance!

[DJGosnell]

I do not believe the library does this currently nor do I think that AutoCad even has this ability included outside of some LSP script or express tools. Representing is fairly simple geometric math with the ellipse equation x²/a² + y²/b² = 1

[bayramakgul]

Sorry, I realize I originally said "ellipse", but what I actually meant was "elliptical arc."
My goal is to approximate an elliptical arc (not a full ellipse) as a polyline. I don’t have any issues with full ellipses — my difficulty is specifically with handling elliptical arcs.

Thanks for pointing that out!

[DomCR]

Hi @bayramakgul,

There is a similar feature request to create a Hatch from entities, I'll open a branch to implement a similar feature but instead to create a Hatch to create a Polyline.

Here is the discussion:

• Create Hatch #466

[bayramakgul]

        // create a new drawing
        CadDocument doc = new CadDocument();

        string filePath = "ellipse_to_polyline_input.dwg";
        FileInfo file = new FileInfo(filePath);
        var dwg_reader = new DwgReader(filePath);


        // read the drawing
        doc = dwg_reader.Read();

        List<Entity> add_entities = new List<Entity>();
        List<Entity> remove_entities = new List<Entity>();

        foreach (var e in doc.Entities)
        {
            switch (e.ObjectType)
            {
                case ObjectType.ELLIPSE:
                    var ellipse = (ACadSharp.Entities.Ellipse)e;

                    // convert the ellipse to a polyline 30 segments
                    var Vertices = ellipse.PolygonalVertexes(30);

                    // create a polyline from the ellipse
                    Polyline2D polyline = new Polyline2D();
                    polyline.LineType = ellipse.LineType;
                    polyline.Color = ellipse.Color;
                    polyline.Layer = ellipse.Layer;
                    polyline.LineWeight = ellipse.LineWeight;
                    polyline.Flags = PolylineFlags.Polyline3D;

                    foreach (var v in Vertices)
                    {
                        polyline.Vertices.Add(new Vertex2D(new CSMath.XYZ(v.X, v.Y, 0)));
                    }

                    // add the polyline to the document
                    add_entities.Add(polyline);

                    // remove the ellipse from the document
                    remove_entities.Add(e);

                    break;
                default:
                    // skip other entities
                    break;
            }
        }


        // remove the old entities from the document
        foreach (var e in remove_entities)
        {
            doc.Entities.Remove(e);
        }

        // add the new entities to the document
        foreach (var e in add_entities)
        {
            doc.Entities.Add(e);
        }

        // save the drawing to a file
        string savePath = $"ellipse_to_polyline_output.dwg";
        using (FileStream fs = new FileStream(savePath, FileMode.Create))
        {
            DwgWriter writer = new DwgWriter(fs, doc);
            writer.Write();
        }

        Console.WriteLine($"file created: {Path.GetFullPath(savePath)}");

ellipse_to_polyline.zip

[DomCR]

for this line, // create a polyline from the ellipse, I would recommend to use MatchProperties() it will simplify your code.

[bayramakgul]

I think this has been solved before??? 😟

Drawing2.zip

`using ACadSharp;
using ACadSharp.Entities;
using ACadSharp.IO;

namespace ConsoleApp12
{
internal class Program
{
static void Main(string[] args)
{
// create a new drawing
CadDocument doc = new CadDocument();

        string filePath = "Drawing2.dwg";
        FileInfo file = new FileInfo(filePath);
        var dwg_reader = new DwgReader(filePath);


        // read the drawing
        doc = dwg_reader.Read();

        List<Entity> add_entities = new List<Entity>();
        List<Entity> remove_entities = new List<Entity>();

        foreach (var e in doc.Entities)
        {
            switch (e.ObjectType)
            {
                case ObjectType.ELLIPSE:
                    var ellipse = (ACadSharp.Entities.Ellipse)e;

                    // convert the ellipse to a polyline 20 segments
                    var Vertices = ellipse.PolygonalVertexes(20);
                    // create a polyline from the ellipse
                    Polyline2D polyline = new Polyline2D();
                    polyline.LineType = ellipse.LineType;
                    polyline.Color = ellipse.Color;
                    polyline.Layer = ellipse.Layer;
                    polyline.LineWeight = ellipse.LineWeight;

                    foreach (var v in Vertices)
                    {
                        polyline.Vertices.Add(new Vertex2D(new CSMath.XYZ(v.X, v.Y, 0)));
                    }

                    // add the polyline to the document
                    add_entities.Add(polyline);

                    // remove the ellipse from the document
                    remove_entities.Add(e);

                    break;
                default:
                    // skip other entities
                    break;
            }
        }


        // remove the old entities from the document
        foreach (var e in remove_entities)
        {
            doc.Entities.Remove(e);
        }

        // add the new entities to the document
        foreach (var e in add_entities)
        {
            doc.Entities.Add(e);
        }

        // save the drawing to a file
        string savePath = $"ellipse_to_polyline_output.dwg";
        using (FileStream fs = new FileStream(savePath, FileMode.Create))
        {
            DwgWriter writer = new DwgWriter(fs, doc);
            writer.Write();
        }

        Console.WriteLine($"file created: {Path.GetFullPath(savePath)}");
    }
}

}
`

[DomCR]

Hi @bayramakgul,

Yes it was fixed, but I tried to modify the method so it works for Arcs and Ellipses, big mistake...

I'll open a PR to fix it, again 😅

Thanks for the report!

[bayramakgul]

@DomCR you're a true master. 😊