Create a rounded rectangle path with ImageSharp

RoundedRectangleExtension.cs

#if !IMAGESHARP_V2
using System;
using System.Collections.Generic;
using System.Numerics;
#endif
using SixLabors.ImageSharp;
using SixLabors.ImageSharp.Drawing;

namespace RoundedRectangleTest
{
    public static class RoundedRectangleExtension
    {
        public static IPath ToRoundedRectangle(this RectangleF rectangle, float cornerRadius) => new PathBuilder()
            .AddEllipticalArc(rectangle.Left + cornerRadius, rectangle.Top + cornerRadius, cornerRadius, cornerRadius, 0, -90, -90)
            .AddEllipticalArc(rectangle.Right - cornerRadius, rectangle.Top + cornerRadius, cornerRadius, cornerRadius, 0, 180, -90)
            .AddEllipticalArc(rectangle.Right - cornerRadius, rectangle.Bottom - cornerRadius, cornerRadius, cornerRadius, 0, 90, -90)
            .AddEllipticalArc(rectangle.Left + cornerRadius, rectangle.Bottom - cornerRadius, cornerRadius, cornerRadius, 0, 0, -90)
            .CloseFigure()
            .Build()
            ;

#if !IMAGESHARP_V2
        private static PathBuilder AddEllipticalArc(this PathBuilder builder, float x, float y, float radiusX, float radiusY, float rotation, float startAngle, float sweepAngle) =>
            builder.AddSegment(new EllipticalArcLineSegment(x, y, radiusX, radiusY, rotation, startAngle, sweepAngle, Matrix3x2.Identity));

        // Copied from https://github.com/SixLabors/ImageSharp.Drawing/blob/7ffae70dfb9eaf8a7e03ed98d8c2e60e0aed2ed7/src/ImageSharp.Drawing/Shapes/EllipticalArcLineSegment.cs
        private sealed class EllipticalArcLineSegment : ILineSegment
        {
            private const float MinimumSqrDistance = 1.75f;
            private readonly PointF[] linePoints;
            private readonly float x;
            private readonly float y;
            private readonly float radiusX;
            private readonly float radiusY;
            private readonly float rotation;
            private readonly float startAngle;
            private readonly float sweepAngle;
            private readonly Matrix3x2 transformation;

            public EllipticalArcLineSegment(float x, float y, float radiusX, float radiusY, float rotation, float startAngle, float sweepAngle, Matrix3x2 transformation)
            {
                this.x = x;
                this.y = y;
                this.radiusX = radiusX;
                this.radiusY = radiusY;
                this.rotation = rotation % 360;
                this.startAngle = startAngle % 360;
                this.transformation = transformation;
                this.sweepAngle = sweepAngle;
                if (sweepAngle > 360)
                    this.sweepAngle = 360;

                if (sweepAngle < -360)
                    this.sweepAngle = -360;

                linePoints = GetDrawingPoints();
                EndPoint = linePoints[linePoints.Length - 1];
            }

            public PointF EndPoint { get; }

            public EllipticalArcLineSegment Transform(Matrix3x2 matrix) => matrix.IsIdentity
                ? this
                : new EllipticalArcLineSegment(x, y, radiusX, radiusY, rotation, startAngle, sweepAngle, Matrix3x2.Multiply(transformation, matrix));

            ILineSegment ILineSegment.Transform(Matrix3x2 matrix) => Transform(matrix);

            private PointF[] GetDrawingPoints()
            {
                var points = new List<PointF>() { CalculatePoint(startAngle) };

                if (sweepAngle < 0)
                {
                    for (var i = startAngle; i > startAngle + sweepAngle; i--)
                    {
                        var end = i - 1;
                        if (end <= startAngle + sweepAngle)
                            end = startAngle + sweepAngle;

                        points.AddRange(GetDrawingPoints(i, end, 0));
                    }
                }
                else
                {
                    for (var i = startAngle; i < startAngle + sweepAngle; i++)
                    {
                        var end = i + 1;
                        if (end >= startAngle + sweepAngle)
                            end = startAngle + sweepAngle;

                        points.AddRange(GetDrawingPoints(i, end, 0));
                    }
                }

                return points.ToArray();
            }

            private List<PointF> GetDrawingPoints(float start, float end, int depth)
            {
                if (depth > 1000)
                    return new List<PointF>();

                var points = new List<PointF>();

                var startP = CalculatePoint(start);
                var endP = CalculatePoint(end);
                if ((new Vector2(endP.X, endP.Y) - new Vector2(startP.X, startP.Y)).LengthSquared() < MinimumSqrDistance)
                    points.Add(endP);
                else
                {
                    float mid = start + (end - start) / 2;
                    points.AddRange(GetDrawingPoints(start, mid, depth + 1));
                    points.AddRange(GetDrawingPoints(mid, end, depth + 1));
                }

                return points;
            }

            private PointF CalculatePoint(float angle)
            {
                var x = radiusX * MathF.Sin(MathF.PI * angle / 180) * MathF.Cos(MathF.PI * rotation / 180) -
                    radiusY * MathF.Cos(MathF.PI * angle / 180) * MathF.Sin(MathF.PI * rotation / 180) + this.x;
                var y = radiusX * MathF.Sin(MathF.PI * angle / 180) * MathF.Sin(MathF.PI * rotation / 180) +
                    radiusY * MathF.Cos(MathF.PI * angle / 180) * MathF.Cos(MathF.PI * rotation / 180) + this.y;

                return PointF.Transform(new PointF(x, y), transformation);
            }

            public ReadOnlyMemory<PointF> Flatten() => linePoints;
        }
#endif
    }
}

Simplified the end of the ToRoundedRectangle by using PathBuilder

NB: Everything inside the #if !IMAGESHARP_V2 can be removed once SixLabors.ImageSharp v2 is released