indexed pixel format 图片格式显示出错

错误描述：A Graphics object cannot be created from an image that has an indexed pixel

 

解决思路：做一个同样尺寸的bitmap，将“indexed pixel format”格式的图片复制过来，变为一个非“indexed pixel format”的图片

 

实现代码：

protected void Page_Load(object sender, EventArgs e)
    {
            if(Page.IsPostBack ==false)
            {
            string file = "mailbanner.gif";
            string addText = "上海星移企业邮局";

            try
            {
                addText = Request.QueryString["mark"].ToString();   
            }
            catch(Exception)
            {
            }

            //载入原始图片
            Bitmap bmp = new Bitmap(Server.MapPath(file));
            Graphics graphics;
            try
            {
                //试图取得Graphics对象，用于添加文字

                //对于indexed pixel format，此处会抛出异常
                graphics = System.Drawing.Graphics.FromImage(bmp);
            }
            catch (Exception)
            {

                //对于indexed pixel format，抛出异常的特殊处理
                Bitmap newBmp = new Bitmap(bmp.Width, bmp.Height);
                graphics = System.Drawing.Graphics.FromImage(newBmp); 
                graphics.DrawImage(bmp,
                                   new Rectangle(0, 0, newBmp.Width, newBmp.Height),
                                   new Rectangle(0, 0, bmp.Width, bmp.Height),
                                   GraphicsUnit.Pixel);
                bmp = newBmp;
            }

            graphics.DrawString(addText,
                new Font("Verdana", 14, FontStyle.Regular ),
                new SolidBrush(Color.Gray), 181, 9);
           
            graphics.DrawString(addText,
                new Font("Verdana", 14, FontStyle.Regular),
                new SolidBrush(Color.Goldenrod  ), 180, 8);

            graphics.Dispose();

            System.IO.MemoryStream ms = new System.IO.MemoryStream();
            //网上的例程，通常是 bmp.Save(ms, System.Drawing.Imaging.ImageFormat.Jpeg)

            //色彩失真明显

            bmp.Save(ms, System.Drawing.Imaging.ImageFormat.Bmp);
            bmp.Dispose();

            Response.ClearContent();
            Response.ContentType = "image/bmp";
            Response.BinaryWrite(ms.ToArray());
        }

    }