无法从带有索引像素格式的图像创建graphics对象

http://www.cnblogs.com/qixuejia/archive/2010/09/03/1817248.html

大家在用 .NET 做图片水印功能的时候， 很可能会遇到 “无法从带有索引像素格式的图像创建graphics对象”这个错误，对应的英文错误提示是“A Graphics object cannot be created from an image that has an indexed pixel format"

这个exception是出现在 System.Drawing.Graphics g = System.Drawing.Graphics.FromImage("图片路径")  

这个调用的语句上，通过查询 MSDN， 我们可以看到如下的提示信息：

通过上面的错误解释，我们可以看到，原因是因为图片是索引像素格式的。为了避免此问题的发生，我们在做水印之前，可以先判断原图片是否是索引像素格式的，如果是，则可以采用将此图片先clone到一张BMP上的方法来解决：

/// <summary>/// 会产生graphics异常的PixelFormat/// </summary>private static PixelFormat[] indexedPixelFormats = { PixelFormat.Undefined, PixelFormat.DontCare,PixelFormat.Format16bppArgb1555, PixelFormat.Format1bppIndexed, PixelFormat.Format4bppIndexed,PixelFormat.Format8bppIndexed    };/// <summary>/// 判断图片的PixelFormat 是否在 引发异常的 PixelFormat 之中/// </summary>/// <param name="imgPixelFormat">原图片的PixelFormat</param>/// <returns></returns>private static bool IsPixelFormatIndexed(PixelFormat imgPixelFormat){    foreach (PixelFormat pf in indexedPixelFormats)    {        if (pf.Equals(imgPixelFormat)) return true;    }    return false;}//.........使用using (Image img = Image.FromFile("原图片路径")){    //如果原图片是索引像素格式之列的，则需要转换    if (IsPixelFormatIndexed(img.PixelFormat))    {        Bitmap bmp = new Bitmap(img.Width, img.Height, PixelFormat.Format32bppArgb);        using (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);        }        //下面的水印操作，就直接对 bmp 进行了        //......    }    else //否则直接操作    {         //直接对img进行水印操作    }}

// <summary>        /// 在图片上生成图片水印        /// </summary>        /// <param name="Path">原服务器图片路径</param>        /// <param name="Path_syp">生成的带图片水印的图片路径</param>        /// <param name="Path_sypf">水印图片路径</param>        public static void AddWaterPic(string Path, string Path_syp, string Path_sypf, double xPer, double yPer)        {            using (Image img = Image.FromFile(Path))            {                //如果原图片是索引像素格式之列的，则需要转换                if (IsPixelFormatIndexed(img.PixelFormat))                {                    Bitmap bmp = new Bitmap(img.Width, img.Height, PixelFormat.Format32bppArgb);                    using (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);                    }                                        System.Drawing.Image copyImage = System.Drawing.Image.FromFile(Path_sypf);                    System.Drawing.Graphics g2 = System.Drawing.Graphics.FromImage(bmp);                    int x = (int)Convert.ToInt32(bmp.Width * xPer);                    int y = (int)Convert.ToInt32(bmp.Height * yPer);
                    g2.DrawImage(copyImage, new System.Drawing.Rectangle(x, y, copyImage.Width, copyImage.Height), 0, 0, copyImage.Width, copyImage.Height, System.Drawing.GraphicsUnit.Pixel);                    bmp.Save(Path_syp);
                    g2.Dispose();                    bmp.Dispose();
                }                else                {                    System.Drawing.Image image = System.Drawing.Image.FromFile(Path);                    System.Drawing.Image copyImage = System.Drawing.Image.FromFile(Path_sypf);                    System.Drawing.Graphics g = System.Drawing.Graphics.FromImage(image);                    int x = (int)Convert.ToInt32(image.Width * xPer);                    int y = (int)Convert.ToInt32(image.Height * yPer);
                    g.DrawImage(copyImage, new System.Drawing.Rectangle(x, y, copyImage.Width, copyImage.Height), 0, 0, copyImage.Width, copyImage.Height, System.Drawing.GraphicsUnit.Pixel);                    image.Save(Path_syp);
                    g.Dispose();                    image.Dispose();                }            }        }        #endregion        #region        /// <summary>        /// 会产生graphics异常的PixelFormat        /// </summary>        private static PixelFormat[] indexedPixelFormats = { PixelFormat.Undefined, PixelFormat.DontCare, PixelFormat.Format16bppArgb1555, PixelFormat.Format1bppIndexed, PixelFormat.Format4bppIndexed, PixelFormat.Format8bppIndexed };
        /// <summary>        /// 判断图片的PixelFormat 是否在 引发异常的 PixelFormat 之中        /// 无法从带有索引像素格式的图像创建graphics对象        /// </summary>        /// <param name="imgPixelFormat">原图片的PixelFormat</param>        /// <returns></returns>        private static bool IsPixelFormatIndexed(PixelFormat imgPixelFormat)        {            foreach (PixelFormat pf in indexedPixelFormats)            {                if (pf.Equals(imgPixelFormat)) return true;            }
            return false;        }
        #endregion