/// summary> 
   /// 為圖片生成縮略圖
   /// /summary> 
   /// param name="phyPath">原圖片的路徑/param> 
   /// param name="width">縮略圖寬/param> 
   /// param name="height">縮略圖高/param> 
   /// returns>/returns> 
   public System.Drawing.Image GetHvtThumbnail(System.Drawing.Image image, int width, int height)
   {
     Bitmap m_hovertreeBmp = new Bitmap(width, height);
     //從Bitmap創建一個System.Drawing.Graphics 
     Graphics m_HvtGr = Graphics.FromImage(m_hovertreeBmp);
     //設置 
     m_HvtGr.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.HighQuality;
     //下面這個也設成高質量 
     m_HvtGr.CompositingQuality = System.Drawing.Drawing2D.CompositingQuality.HighQuality;
     //下面這個設成High 
     m_HvtGr.InterpolationMode = System.Drawing.Drawing2D.InterpolationMode.HighQualityBicubic;
     //把原始圖像繪制成上面所設置寬高的縮小圖 
     Rectangle rectDestination = new Rectangle(0, 0, width, height);

     int m_width, m_height;
     if (image.Width * height > image.Height * width)
     {
       m_height = image.Height;
       m_width = (image.Height * width) / height;
     }
     else
     {
       m_width = image.Width;
       m_height = (image.Width * height) / width;
     }

     m_HvtGr.DrawImage(image, rectDestination, 0, 0, m_width, m_height, GraphicsUnit.Pixel);

     return m_hovertreeBmp;
   }

using System;
using System.Collections.Generic;
using System.Drawing;
using System.Drawing.Imaging;
namespace HtmlSnap
{
  public static class ImageHelper
  {
    /// 獲取縮略圖
    public static Image GetThumbnailImage(Image image, int width, int height)
    {
      if (image == null || width  1 || height  1)
        return null;
      // 新建一個bmp圖片
      Image bitmap = new System.Drawing.Bitmap(width, height);

      // 新建一個畫板
      using (Graphics g = System.Drawing.Graphics.FromImage(bitmap))
      {

        // 設置高質量插值法
        g.InterpolationMode = System.Drawing.Drawing2D.InterpolationMode.HighQualityBicubic;

        // 設置高質量,低速度呈現平滑程度
        g.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.HighQuality;
        
        // 高質量、低速度復合
        g.CompositingQuality = System.Drawing.Drawing2D.CompositingQuality.HighQuality;
        
 // 清空畫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繪制原圖片的指定部分
        g.DrawImage(image, new Rectangle(0, 0, width, height),
          new Rectangle(0, 0, image.Width, image.Height),
          GraphicsUnit.Pixel);
        return bitmap;
      }
    }
    /// summary>
    /// 生成縮略圖，并保持縱橫比
    /// /summary>
    /// returns>生成縮略圖后對象/returns>
    public static Image GetThumbnailImageKeepRatio(Image image, int width, int height)
    {
      Size imageSize = GetImageSize(image, width, height);
      return GetThumbnailImage(image, imageSize.Width, imageSize.Height);
    }

    /// summary>
    /// 根據百分比獲取圖片的尺寸
    /// /summary>
    public static Size GetImageSize(Image picture, int percent)
    {
      if (picture == null || percent  1)
        return Size.Empty;

      int width = picture.Width * percent / 100;
      int height = picture.Height * percent / 100;

      return GetImageSize(picture, width, height);
    }
    /// summary>
    /// 根據設定的大小返回圖片的大小，考慮圖片長寬的比例問題
    /// /summary>
    public static Size GetImageSize(Image picture, int width, int height)
    {
      if (picture == null || width  1 || height  1)
        return Size.Empty;
      Size imageSize;
      imageSize = new Size(width, height);
      double heightRatio = (double)picture.Height / picture.Width;
      double widthRatio = (double)picture.Width / picture.Height;
      int desiredHeight = imageSize.Height;
      int desiredWidth = imageSize.Width;
      imageSize.Height = desiredHeight;
      if (widthRatio > 0)
        imageSize.Width = Convert.ToInt32(imageSize.Height * widthRatio);
      if (imageSize.Width > desiredWidth)
      {
        imageSize.Width = desiredWidth;
        imageSize.Height = Convert.ToInt32(imageSize.Width * heightRatio);
      }
      return imageSize;
    }
    /// summary>
    /// 獲取圖像編碼解碼器的所有相關信息
    /// /summary>
    /// param name="mimeType">包含編碼解碼器的多用途網際郵件擴充協議 (MIME) 類型的字符串/param>
    /// returns>返回圖像編碼解碼器的所有相關信息/returns>
    public static ImageCodecInfo GetCodecInfo(string mimeType)
    {
      ImageCodecInfo[] CodecInfo = ImageCodecInfo.GetImageEncoders();
      foreach (ImageCodecInfo ici in CodecInfo)
      {
        if (ici.MimeType == mimeType) return ici;
      }
      return null;
    }
    public static ImageCodecInfo GetImageCodecInfo(ImageFormat format)
    {
      ImageCodecInfo[] encoders = ImageCodecInfo.GetImageEncoders();
      foreach (ImageCodecInfo icf in encoders)
      {
        if (icf.FormatID == format.Guid)
        {
          return icf;
        }
      }
      return null;
    }
    public static void SaveImage(Image image, string savePath, ImageFormat format)
    {
      SaveImage(image, savePath, GetImageCodecInfo(format));
    }
    /// summary>
    /// 高質量保存圖片
    /// /summary>
    private static void SaveImage(Image image, string savePath, ImageCodecInfo ici)
    {
      // 設置 原圖片 對象的 EncoderParameters 對象
      EncoderParameters parms = new EncoderParameters(1);
      EncoderParameter parm = new EncoderParameter(Encoder.Quality, ((long)95));
      parms.Param[0] = parm;
      image.Save(savePath, ici, parms);
      parms.Dispose();
    }

  }
}