C#写的透明背景验证码

protected void Page_Load(object sender, EventArgs e)
    {
        string valueStr = "";
        int fontsize = 20;
        int count = 5;
        int fwidth = -2;//字体间的距离
        int width = (fontsize + fwidth) * count + 5;
        int height = fontsize + 6;

        Font f;
        Random r = new Random();
        string vs = "abcdefghjkmnpqrstuvwxyzABCDEFGHJKLMNPQRSTUVWXYZ23456789";
        string fs = "宋体,新宋体,仿宋_GB2312,黑体,楷体_GB2312";
        Color[] cs = new Color[] { Color.White, Color.White, Color.Yellow };
        System.Drawing.Image bitmap = new System.Drawing.Bitmap(width, height, System.Drawing.Imaging.PixelFormat.Format32bppArgb);
        //System.Drawing.Image bitmap = new System.Drawing.Bitmap(width, height);//不透明
        Graphics graphics = Graphics.FromImage(bitmap);
        //graphics.Clear(Color.FromArgb(60, 107, 139));//不透明
        graphics.Clear(Color.Transparent);

        for (int i = 0; i < count; i++)
        {
            string cha = vs.Substring(r.Next(vs.Length - 1), 1);
            valueStr += cha;
            f = new Font(fs.Split(',')[r.Next(fs.Split(',').Length)], fontsize, FontStyle.Bold);
            graphics.DrawString(cha, f, new SolidBrush(cs[r.Next(cs.Length)]), new PointF(i * (fontsize + fwidth), 0));
        }

        Session.Add("validatStr", valueStr);