.net等比缩放生成缩略图的方法

生成缩略图是一个十分常用功能，找到了一个方法，重写部分代码，实用又好用，.net又一个生成缩略图的方法，不变形

/// <summary> 
   /// 为图片生成缩略图
   /// </summary> 
   /// <param name="phypath">原图片的路径</param> 
   /// <param name="width">缩略图宽</param> 
   /// <param name="height">缩略图高</param> 
   /// <returns></returns> 
   public system.drawing.image gethvtthumbnail(system.drawing.image image, int width, int height)
   {
     bitmap m_hovertreebmp = new bitmap(width, height);
     //从bitmap创建一个system.drawing.graphics 
     graphics m_hvtgr = graphics.fromimage(m_hovertreebmp);
     //设置 
     m_hvtgr.smoothingmode = system.drawing.drawing2d.smoothingmode.highquality;
     //下面这个也设成高质量 
     m_hvtgr.compositingquality = system.drawing.drawing2d.compositingquality.highquality;
     //下面这个设成high 
     m_hvtgr.interpolationmode = system.drawing.drawing2d.interpolationmode.highqualitybicubic;
     //把原始图像绘制成上面所设置宽高的缩小图 
     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;
   }

c#缩略图生成类，采用高质量插值法实现缩略图生成，高质量,低速度呈现平滑程度，可以保持缩略图纵横比，在获取缩略图的时候一开始就根据百分比获取图片的尺寸、根据设定的大小返回图片的大小，并高质量保存缩略图图片，为原图片设置encoderparameters 对象。

以下为类文件，建议保存文件名为：imagehelper.cs

using system;
using system.collections.generic;
using system.drawing;
using system.drawing.imaging;
namespace htmlsnap
{
  public static class imagehelper
  {
    /// 获取缩略图
    public static image getthumbnailimage(image image, int width, int height)
    {
      if (image == null || width < 1 || height < 1)
        return null;
      // 新建一个bmp图片
      image bitmap = new system.drawing.bitmap(width, height);

      // 新建一个画板
      using (graphics g = system.drawing.graphics.fromimage(bitmap))
      {

        // 设置高质量插值法
        g.interpolationmode = system.drawing.drawing2d.interpolationmode.highqualitybicubic;

        // 设置高质量,低速度呈现平滑程度
        g.smoothingmode = system.drawing.drawing2d.smoothingmode.highquality;
        
        // 高质量、低速度复合
        g.compositingquality = system.drawing.drawing2d.compositingquality.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(image, new rectangle(0, 0, width, height),
          new rectangle(0, 0, image.width, image.height),
          graphicsunit.pixel);
        return bitmap;
      }
    }
    /// <summary>
    /// 生成缩略图，并保持纵横比
    /// </summary>
    /// <returns>生成缩略图后对象</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>
    /// 根据百分比获取图片的尺寸
    /// </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>
    /// 根据设定的大小返回图片的大小，考虑图片长宽的比例问题
    /// </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>
    /// 获取图像编码解码器的所有相关信息
    /// </summary>
    /// <param name="mimetype">包含编码解码器的多用途网际邮件扩充协议 (mime) 类型的字符串</param>
    /// <returns>返回图像编码解码器的所有相关信息</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>
    /// 高质量保存图片
    /// </summary>
    private static void saveimage(image image, string savepath, imagecodecinfo ici)
    {
      // 设置 原图片 对象的 encoderparameters 对象
      encoderparameters parms = new encoderparameters(1);
      encoderparameter parm = new encoderparameter(encoder.quality, ((long)95));
      parms.param[0] = parm;
      image.save(savepath, ici, parms);
      parms.dispose();
    }

  }
}