asp.net 验证码文字旋转

            Bitmap image = new Bitmap(250, 250);

            Graphics g = Graphics.FromImage(image);

            StringFormat format = new StringFormat(StringFormatFlags.NoClip);

            Random r = new Random();
            Font font = new System.Drawing.Font("SimSun", 12);
            System.Drawing.Drawing2D.LinearGradientBrush brush = new System.Drawing.Drawing2D.LinearGradientBrush(new Rectangle(0, 0, image.Width, image.Height), Color.Blue, Color.DarkRed, 1.2f, true);
            g.Clear(Color.White);
            g.TextRenderingHint = System.Drawing.Text.TextRenderingHint.AntiAliasGridFit;

            PointF rotatePoint = new PointF(0, 0);
            int a = 1;
            Matrix myMatrix = new Matrix();
            int rintrotate = 0;
            for (int i = 0; i < 5; i++)
            {
                a = a + 12;

               
                rintrotate  = r.Next(-15,20);
                //myMatrix.Rotate(15);
                myMatrix.RotateAt(rintrotate, new PointF(a, 12));
                g.Transform = myMatrix;


                g.DrawString("a", font, brush, a, 2, format);
                myMatrix.RotateAt(rintrotate*(-1), new PointF(a, 12)); ;
                g.Transform = myMatrix;
            }

            //g.DrawString("abcdefghijklimn", font, brush, a, 2, format);

            g.Transform = new Matrix();
            g.DrawRectangle(new Pen(Color.Silver), 0, 0, image.Width - 1, image.Height - 1);

            System.IO.MemoryStream ms = new System.IO.MemoryStream();
            image.Save(ms, System.Drawing.Imaging.ImageFormat.Gif);
            Response.ClearContent();
            Response.ContentType = "image/Gif";
            Response.BinaryWrite(ms.ToArray());

            g.Dispose();
            image.Dispose();