/// <summary> 
    /// 生成缩略图:背景颜色可以设置
    /// </summary> 
    /// <param name="originalImagePath">源图路径（物理路径）</param> 
    /// <param name="thumbnailPath">缩略图路径（物理路径）</param> 
    /// <param name="width">缩略图宽度</param>
    /// <param name="height">缩略图高度</param>
    /// <param name="byColor">空白背景填充颜色</param>
    public static void MakeThumbnail(string originalImagePath, string thumbnailPath, int width, int height, string byColor)
    {
        //获取原始图片  
        System.Drawing.Image originalImage = System.Drawing.Image.FromFile(originalImagePath);
        //缩略图画布宽高  
        int towidth = width;
        int toheight = height;
        //原始图片写入画布坐标和宽高(用来设置裁减溢出部分)  
        int x = 0;
        int y = 0;
        int ow = originalImage.Width;
        int oh = originalImage.Height;
        //原始图片画布,设置写入缩略图画布坐标和宽高(用来原始图片整体宽高缩放)  
        int bg_x = 0;
        int bg_y = 0;
        int bg_w = towidth;
        int bg_h = toheight;
        //倍数变量  
        double multiple = 0;
        //获取宽长的或是高长与缩略图的倍数  
        if (originalImage.Width >= originalImage.Height)
        {
            multiple = (double)originalImage.Width / (double)width;
        }
        else
        {
            multiple = (double)originalImage.Height / (double)height;
        }
        //上传的图片的宽和高小等于缩略图  
        if (ow <= width && oh <= height)
        {
            //缩略图按原始宽高  
            bg_w = originalImage.Width;
            bg_h = originalImage.Height;
            //空白部分用背景色填充  
            bg_x = Convert.ToInt32(((double)towidth - (double)ow) / 2);
            bg_y = Convert.ToInt32(((double)toheight - (double)oh) / 2);
        }
        //上传的图片的宽和高大于缩略图  
        else
        {
            //宽高按比例缩放  
            bg_w = Convert.ToInt32((double)originalImage.Width / multiple);
            bg_h = Convert.ToInt32((double)originalImage.Height / multiple);
            //空白部分用背景色填充  
            bg_y = Convert.ToInt32(((double)height - (double)bg_h) / 2);
            bg_x = Convert.ToInt32(((double)width - (double)bg_w) / 2);
        }
        //新建一个bmp图片,并设置缩略图大小.  
        System.Drawing.Image bitmap = new System.Drawing.Bitmap(towidth, toheight);
        //新建一个画板  
        System.Drawing.Graphics g = System.Drawing.Graphics.FromImage(bitmap);
        //设置高质量插值法  
        g.InterpolationMode = System.Drawing.Drawing2D.InterpolationMode.HighQualityBilinear;
        //设置高质量,低速度呈现平滑程度  
        g.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.HighQuality;
        //清空画布并设置背景色  
        g.Clear(System.Drawing.ColorTranslator.FromHtml(byColor));
        //在指定位置并且按指定大小绘制原图片的指定部分  
        //第一个System.Drawing.Rectangle是原图片的画布坐标和宽高,第二个是原图片写在画布上的坐标和宽高,最后一个参数是指定数值单位为像素  
        g.DrawImage(originalImage, new System.Drawing.Rectangle(bg_x, bg_y, bg_w, bg_h), new System.Drawing.Rectangle(x, y, ow, oh), System.Drawing.GraphicsUnit.Pixel);
        try
        {
            //获取图片类型  
            string fileExtension = System.IO.Path.GetExtension(originalImagePath).ToLower();
            //按原图片类型保存缩略图片,不按原格式图片会出现模糊,锯齿等问题.  
            switch (fileExtension)
            {
                case ".gif": bitmap.Save(thumbnailPath, System.Drawing.Imaging.ImageFormat.Gif); break;
                case ".jpg": bitmap.Save(thumbnailPath, System.Drawing.Imaging.ImageFormat.Jpeg); break;
                case ".bmp": bitmap.Save(thumbnailPath, System.Drawing.Imaging.ImageFormat.Bmp); break;
                case ".png": bitmap.Save(thumbnailPath, System.Drawing.Imaging.ImageFormat.Png); break;
            }
        }
        catch (System.Exception e)
        {
            throw e;
        }
        finally
        {
            originalImage.Dispose();
            bitmap.Dispose();
            g.Dispose();
        }
    }


    /// <summary> 
    /// 生成缩略图：无填充背景
    /// </summary> 
    /// <param name="originalImagePath">源图路径（物理路径）</param> 
    /// <param name="thumbnailPath">缩略图路径（物理路径）</param> 
    /// <param name="width">缩略图宽度</param> 
    /// <param name="height">缩略图高度</param>    
    public static void MakeThumbnail2(string originalImagePath, string thumbnailPath, int width, int height)
    {
        //获取原始图片
        System.Drawing.Image originalImage = System.Drawing.Image.FromFile(originalImagePath);
        //缩略图画布宽高
        int towidth = width;//缩略图宽度
        int toheight = height;//缩略图高度
        //原始图片写入画布坐标和宽高(用来设置裁减溢出部分)
        int x = 0;
        int y = 0;
        int ow = originalImage.Width;//原图宽度
        int oh = originalImage.Height;//原图高度

        //先判断宽，宽大于指定宽度的话将宽设置为指定宽度，高度等比例缩放。
        if (originalImage.Width > width)
        {
            towidth = width;
            toheight = originalImage.Height * towidth / originalImage.Width;
        }
        else
        {
            towidth = originalImage.Width;
            toheight = originalImage.Height;
        }
        //再判断高，高大于指定高度的话将高设置为指定高度，宽度等比例缩放。
        if (toheight > height)
        {
            toheight = height;
            towidth = originalImage.Width * toheight / originalImage.Height;
        }

        //新建一个bmp图片 
        System.Drawing.Image bitmap = new System.Drawing.Bitmap(towidth, toheight);

        //新建一个画板 
        Graphics g = System.Drawing.Graphics.FromImage(bitmap);

        //设置高质量插值法 
        g.InterpolationMode = System.Drawing.Drawing2D.InterpolationMode.High;

        //设置高质量,低速度呈现平滑程度 
        g.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.HighQuality;

        //清空画布并以透明背景色填充 
        g.Clear(Color.Transparent);

        //在指定位置并且按指定大小绘制原图片的指定部分
        g.DrawImage(originalImage, new Rectangle(0, 0, towidth, toheight), new Rectangle(x, y, ow, oh),
GraphicsUnit.Pixel);

        try
        {
            //获取图片类型
            string fileExtension = System.IO.Path.GetExtension(originalImagePath).ToLower();
            //按原图片类型保存缩略图片,不按原格式图片会出现模糊,锯齿等问题.
            switch (fileExtension)
            {
                case ".gif": bitmap.Save(thumbnailPath, System.Drawing.Imaging.ImageFormat.Gif); break;
                case ".jpg": bitmap.Save(thumbnailPath, System.Drawing.Imaging.ImageFormat.Jpeg); break;
                case ".bmp": bitmap.Save(thumbnailPath, System.Drawing.Imaging.ImageFormat.Bmp); break;
                case ".png": bitmap.Save(thumbnailPath, System.Drawing.Imaging.ImageFormat.Png); break;
            }
        }
        catch (System.Exception e)
        {
            throw e;
        }
        finally
        {
            originalImage.Dispose();
            bitmap.Dispose();
            g.Dispose();
        }
    }

protected void Page_Load(object sender, EventArgs e)
        {
            //测试
            string picUrl = "/upload/1.png";
            string picPath = Server.MapPath(picUrl);

            ChangePicSize(picPath, 800);
            Response.Write("<img src='" + picUrl + "'>");
        }
        
        /// <summary>
        /// 等比缩放图片
        /// </summary>
        /// <param name="pathImageFrom">图片地址（服务器绝对路径）</param>
        /// <param name="maxSize">宽和高最大值</param>
        public void ChangePicSize(string pathImageFrom, int maxSize)
        {
            System.Drawing.Image imageFrom = null;
            if (File.Exists(pathImageFrom))
            {
                imageFrom = System.Drawing.Image.FromFile(pathImageFrom);
                // 源图宽度及高度   
                int imageFromWidth = imageFrom.Width;
                int imageFromHeight = imageFrom.Height;
                // 生成的缩略图实际宽度及高度  
                int bitmapWidth = imageFromWidth;
                int bitmapHeight = imageFromHeight;
                if (imageFromWidth > maxSize || imageFromHeight > maxSize)
                {
                    if (imageFromWidth > imageFromHeight)
                    {
                        bitmapWidth = maxSize;
                        bitmapHeight = imageFromHeight * maxSize / imageFromWidth;
                    }
                    else
                    {
                        bitmapWidth = imageFromWidth * maxSize / imageFromHeight;
                        bitmapHeight = maxSize;
                    }
                }
                // 创建画布   
                System.Drawing.Bitmap bmp = new System.Drawing.Bitmap(bitmapWidth, bitmapHeight);
                System.Drawing.Graphics g = System.Drawing.Graphics.FromImage(bmp);
                // 用白色清空   
                g.Clear(System.Drawing.Color.White);
                // 指定高质量的双三次插值法。执行预筛选以确保高质量的收缩。此模式可产生质量最高的转换图像。
                g.InterpolationMode = System.Drawing.Drawing2D.InterpolationMode.HighQualityBicubic;
                // 指定高质量、低速度呈现。   
                g.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.HighQuality;
                // 在指定位置并且按指定大小绘制指定的 Image 的指定部分。   
                g.DrawImage(imageFrom, new System.Drawing.Rectangle(0, 0, bitmapWidth, bitmapHeight), new System.Drawing.Rectangle(0, 0, imageFromWidth, imageFromHeight), System.Drawing.GraphicsUnit.Pixel);

                imageFrom.Dispose();
                bmp.Save(pathImageFrom, System.Drawing.Imaging.ImageFormat.Jpeg);

                //释放资源     
                bmp.Dispose();
                g.Dispose();
            }
        }