/// <summary>
/// 生成缩略图
/// </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="mode">生成缩略图的方式</param> 
public static void MakeThumbnail(string originalImagePath, string thumbnailPath, int width, int height, string mode)
{
 Image originalImage = Image.FromFile(originalImagePath);
 int towidth = width;
 int toheight = height;
 int x = 0;
 int y = 0;
 int ow = originalImage.Width;
 int oh = originalImage.Height; 
 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();
 }
}