**//**////

            switch (mode)
            {       
                case "HW"://指定高宽缩放（可能变形）               
                    break;
                case "W"://指定宽，高按比例                   
                    toheight = originalImage.Height * width/originalImage.Width;
                    break;
                case "H"://指定高，宽按比例
                    towidth = originalImage.Width * height/originalImage.Height;                   
                    break;       
                case "Cut"://指定高宽裁减（不变形）               
                    if((double)originalImage.Width/(double)originalImage.Height > (double)towidth/(double)toheight)
                    {
                        oh = originalImage.Height;
                        ow = originalImage.Height*towidth/toheight;
                        y = 0;
                        x = (originalImage.Width - ow)/2;
                    }
                    else
                    {
                        ow = originalImage.Width;
                        oh = originalImage.Width*height/towidth;
                        x = 0;
                        y = (originalImage.Height - oh)/2;
                    }
                    break;                   
                default :
                    break;
            }   
           
            //新建一个bmp图片
            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
            {           
                //以jpg格式保存缩略图
                bitmap.Save(thumbnailPath, System.Drawing.Imaging.ImageFormat.Jpeg);
            }
            catch(System.Exception e)
            {
                throw e;
            }
            finally
            {
                originalImage.Dispose();
                bitmap.Dispose();                       
                g.Dispose();
            }
        }