Using matrix transforms for ellipse arcs.

.cs

public static DVec.Vector2[] EllipseFullMtxSweep(double semiMaj, double eccentricity, double tilt, double start,
    double sweep, int numPoints)
{

    //convert ellipse angles to circle angles. 
    double b = semiMaj * Math.Sqrt(1 - eccentricity * eccentricity);
    start = Math.Atan(semiMaj / b * Math.Tan(start));
    sweep = Math.Atan(semiMaj / b * Math.Tan(sweep));
    double end = start + sweep;
    double Δθ = 2 * Math.PI / (numPoints - 1) * Math.Sign(sweep); //arc increment for a whole circle
    numPoints = (int)Math.Abs(sweep / Δθ); //numpoints for just the arc
    DVec.Vector2[] points = new DVec.Vector2[numPoints + 1];

    double linEcc = Math.Sqrt(semiMaj * semiMaj - b * b);
    double θ = 0;
    double x = 0;
    double y = 0;

    for (int i = 0; i < numPoints; i++)
    {
        θ = start + Δθ * i;
        x = semiMaj * Math.Cos(θ);
        y = semiMaj * Math.Sin(θ);

        points[i] = new DVec.Vector2(x, y);
    }
    x = semiMaj * Math.Cos(end);
    y = semiMaj * Math.Sin(end);
    points[numPoints] = new DVec.Vector2(x, y);

    Matrix mirMtx = Matrix.IDMirror(true, false);
    Matrix scalemtx = Matrix.IDScale(1,  1 - eccentricity);
    Matrix moveMtx = Matrix.IDTranslate(-linEcc, 0);
    Matrix rotMtx = Matrix.IDRotate(tilt);
    Matrix endMtx = mirMtx * moveMtx * scalemtx * rotMtx;
    points = endMtx.Transform(points);

    return points;
}