System.Drawing.Drawing2D.Matrix.Rotate C# (CSharp) Method

        public void Rotate(float angle)
        {
            Rotate(angle, MatrixOrder.Prepend);
        }

        internal static float ConvertSweepAngle(float sweepAngle, float startAngle, SpatialTransform transform, CoordinateSystem targetSystem)
        {
            PointF x = new PointF(100, 0);

            PointF[] startVector = new PointF[] { x };
            Matrix   rotation    = new Matrix();

            rotation.Rotate(startAngle);
            rotation.TransformVectors(startVector);

            PointF[] sweepVector = (PointF[])startVector.Clone();
            rotation.Reset();
            rotation.Rotate(sweepAngle);
            rotation.TransformVectors(sweepVector);
            rotation.Dispose();

            SizeF startVectorTransformed, sweepVectorTransformed;

            if (targetSystem == Graphics.CoordinateSystem.Destination)
            {
                startVectorTransformed = transform.ConvertToDestination(new SizeF(startVector[0]));
                sweepVectorTransformed = transform.ConvertToDestination(new SizeF(sweepVector[0]));
            }
            else
            {
                startVectorTransformed = transform.ConvertToSource(new SizeF(startVector[0]));
                sweepVectorTransformed = transform.ConvertToSource(new SizeF(sweepVector[0]));
            }

            // simply return the angle between the start and sweep angle, in the target system.
            return((int)Math.Round(Vector.SubtendedAngle(sweepVectorTransformed.ToPointF(), PointF.Empty, startVectorTransformed.ToPointF())));
        }