Bitmap添加水印提示【無法從帶有索引像素格式的圖像創建Graphics對象】

給圖片添加水印時

Bitmap img = bm.ToBitmap();

Graphics g1 = Graphics.FromImage(img);

調用上面的語句，出現這個錯誤提示：

		Bitmap img = bm.ToBitmap();
               //如果原圖片是索引像素格式之列的，則需要轉換
               if (IsPixelFormatIndexed(img.PixelFormat))
               {
                   Bitmap bmp = new Bitmap(img.Width, img.Height, PixelFormat.Format32bppArgb);
                   Graphics g = Graphics.FromImage(bmp);
                   g.InterpolationMode = System.Drawing.Drawing2D.InterpolationMode.HighQualityBicubic;
                   g.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.HighQuality;
                   g.CompositingQuality = System.Drawing.Drawing2D.CompositingQuality.HighQuality;
                   g.DrawImage(img, 0, 0);
 
                   //加水印
                   String str = "hello, string";
                   Font font = new Font("宋體", 8);
                   SolidBrush sbrush = new SolidBrush(Color.Black);
                   g.DrawString(str, font, sbrush, new PointF(10, 10));
 
                   pictureBox1.Image = bmp;
               }
               else
               {
                   //直接加水印
               }

添加引用和函數

	using System.Drawing.Imaging;
 
        /// <summary>
        /// 會產生graphics異常的PixelFormat
        /// </summary>
        private static PixelFormat[] indexedPixelFormats = {
            PixelFormat.Undefined,
            PixelFormat.DontCare,
            PixelFormat.Format16bppArgb1555,
            PixelFormat.Format1bppIndexed,
            PixelFormat.Format4bppIndexed,
            PixelFormat.Format8bppIndexed
        };
 
        /// <summary>
        /// 判斷圖片的PixelFormat 是否在 引發異常的 PixelFormat 之中
        /// </summary>
        /// <param name="imgPixelFormat">原圖片的PixelFormat</param>
        /// <returns></returns>
        private static bool IsPixelFormatIndexed(PixelFormat imgPixelFormat)
        {
            foreach (PixelFormat pf in indexedPixelFormats)
            {
                if (pf.Equals(imgPixelFormat)) return true;
            }
 
            return false;
        }