Parse incomplete ellipses from DXF files

Question

I am developing a DXF parser by using the dxflib library. I have a problem parsing ellipses.

When I parse an ellipse I receive the following data:

struct DL_EllipseData 
{
    /*! X Coordinate of center point. */
    double cx;
    /*! Y Coordinate of center point. */
    double cy;

    /*! X coordinate of the endpoint of the major axis. */
    double mx;
    /*! Y coordinate of the endpoint of the major axis. */
    double my;

    /*! Ratio of minor axis to major axis. */
    double ratio;
    /*! Startangle of ellipse in rad. */
    double angle1;
    /*! Endangle of ellipse in rad. */
    double angle2;
};

The problem is that when angle1 != 0 AND angle2 != 2* Math.PI the ellipse is open and I am not able to compute the arc path that represents that geometry.

For example considering the following ellipse:

enter image description here

These are its properties:

enter image description here

As you can see the angles are:

angle1 = 0.81855 // 46 degrees
angle2 = 2.38934 // 136 degrees

but the picture of the ellipse (I took from autocad seems to have quite different angles).

Following the section of the DXF file that represents the ellipse:

0
ELLIPSE
  5
A7
330
1F
100
AcDbEntity
  8
DL-Wall
 48
25.0
370
    -3
100
AcDbEllipse
 10
906.6576677029225
 20
906.657675539829
 30
0.0
 11
-641.4561777354752
 21
641.4561777354752
 31
0.0
210
0.0
220
0.0
230
1.0
 40
0.9999999999999978
 41
0.8185500151218715
 42
2.389346341916759

How I have to compute the proper angles (and also start end end point of the arc segment)?

score 1 · Answer 1 · answered Jun 30 '15 at 20:20

    /// <summary>
    ///     Static method to detemine the WPF end point of the arc.
    /// </summary>
    /// <param name="arc">The DXF ArcEntity object to evaluate</param>
    /// <returns>Point with values of the arc end point.</returns>
    public static Point calcEndPoint(Arc arc)
    {
        double x = (Math.Cos(arc.endAngle * (Math.PI / 180)) * arc.radius) + arc.center.X;
        double y = arc.center.Y - (Math.Sin(arc.endAngle * (Math.PI / 180)) * arc.radius);
        return new Point(x, y);
    }

    /// <summary>
    ///     Static method to detemine the WPF start point of the arc.
    /// </summary>
    /// <param name="arc">The DXF ArcEntity object to evaluate</param>
    /// <returns>Point with values of the arc start point.</returns>
    public static Point calcStartPoint(Arc arc)
    {
        double x = (Math.Cos(arc.startAngle * (Math.PI / 180)) * arc.radius) + arc.center.X;
        double y = arc.center.Y - (Math.Sin(arc.startAngle * (Math.PI / 180)) * arc.radius);
        return new Point(x, y);
    }

    public static bool checkArcSize(Arc arc)
    {
        double angleDiff = arc.endAngle - arc.startAngle;
        return !((angleDiff > 0 && angleDiff <= 180) || angleDiff <= -180);
    }

Using System.Windows.Media.Path, PathFigure, ArcSegment:

PathFigure figure = new PathFigure();
figure.StartPoint = calcStartPoint([your arc struct]);
ArcSegment arc = new ArcSegment();
arc.Point = calcEndPoints([your arc struct]);
arc.RotationAngle = Math.Abs(endAngle - startAngle);
arc.Size = new Size(radius, radius);
arc.IsLargeArc = checkArcSize([your arc struct]);
figure.Segments.Add(arc);

You will need to adjust each of the helper functions to use your struct as well as any C++ vs C# differences in syntax.

I believe that these helper functions will bridge the gap between what the DXF spec gives you and what is required to draw the Arc in WPF.

Parse incomplete ellipses from DXF files

1 Answers1