C#2D矩阵旋转

我试着逆时针旋转一条简单的线。但在计算之后,Y坐标总是负值。这是我的代码:

double  degree = 0.785;
       // degree = Convert.ToInt32(degree * Math.PI / 180);
        Graphics g = this.CreateGraphics();

        // Create pen.
        Pen blackPen = new Pen(Color.Black, 3);
        Pen redPen = new Pen(Color.Red, 3);


        // Create points that define line.
        System.Drawing.Point point1 = new System.Drawing.Point(500, 0);
        System.Drawing.Point point2 = new System.Drawing.Point(500, 100);

        // Draw line to screen.
        g.DrawLine(blackPen, point1, point2);
        blackPen.Dispose();

        //Draw ´new Line

        Vector vector1 = new Vector(point2.X, point2.Y);
        Matrix matrix1 = new Matrix(Math.Cos(degree), -Math.Sin(degree), Math.Sin(degree), Math.Cos(degree),0,0);

        Vector result = Vector.Multiply(vector1, matrix1);

        g.DrawLine(redPen,point1.X ,point1.Y,Convert.ToInt32(result.X),Convert.ToInt32(result.Y));

现在我使用as来解决旋转的问题:

double  degree = 45;

matrix.RotateAt(degree, point1.X, point1.Y);