图片无损缩放类（小弟初学，到低有没有损耗，还需高人指点）

using System;
using System.IO;
using System.Drawing;
using System.Drawing.Imaging;
using System.Drawing.Drawing2D;

namespace ZHT.Utility.Share
{
public class Thumbnail
{
   public Thumbnail()
   {
   
   }   

   /// <SUMMARY>
   /// 图片缩放
   /// </SUMMARY>
   /// <PARAM name="sourceFile">图片源路径</PARAM>
   /// <PARAM name="destFile">缩放后图片输出路径</PARAM>
   /// <PARAM name="destHeight">缩放后图片高度</PARAM>
   /// <PARAM name="destWidth">缩放后图片宽度</PARAM>
   /// <RETURNS></RETURNS>
        public static bool GetThumbnail(string sourceFile, string destFile, int destHeight, int destWidth)
        {
            System.Drawing.Image imgSource = System.Drawing.Image.FromFile(sourceFile);
            System.Drawing.Imaging.ImageFormat thisFormat = imgSource.RawFormat;
    int sW = 0, sH = 0; 
            // 按比例缩放
            int sWidth = imgSource.Width;
            int sHeight = imgSource.Height;

    if(sHeight>destHeight || sWidth>destWidth)
    {
     if((sWidth*destHeight)>(sHeight*destWidth))
     {
      sW = destWidth;
      sH = (destWidth*sHeight)/sWidth;
     }
     else
     {
      sH = destHeight;
      sW = (sWidth*destHeight)/sHeight;
     }
    }
    else
    {
     sW = sWidth;
     sH = sHeight;
    }

            Bitmap outBmp = new Bitmap(destWidth, destHeight);
            Graphics g = Graphics.FromImage(outBmp);
    g.Clear(Color.WhiteSmoke);

            // 设置画布的描绘质量
            g.CompositingQuality = CompositingQuality.HighQuality; 
            g.SmoothingMode = SmoothingMode.HighQuality; 
            g.InterpolationMode = InterpolationMode.HighQualityBicubic;

            g.DrawImage(imgSource, new Rectangle((destWidth-sW)/2,(destHeight-sH)/2, sW, sH),0,0, imgSource.Width, imgSource.Height, GraphicsUnit.Pixel);
            g.Dispose();

            // 以下代码为保存图片时，设置压缩质量
            EncoderParameters encoderParams = new EncoderParameters();
            long[] quality = new long[1];
            quality[0] = 100;

            EncoderParameter encoderParam = new EncoderParameter(System.Drawing.Imaging.Encoder.Quality, quality);
            encoderParams.Param[0] = encoderParam;

            try
            {
                //获得包含有关内置图像编码解码器的信息的ImageCodecInfo 对象。
                ImageCodecInfo[] arrayICI = ImageCodecInfo.GetImageEncoders();
                ImageCodecInfo jpegICI = null;
                for (int x = 0; x < arrayICI.Length; x++)
                {
                    if (arrayICI[x].FormatDescription.Equals("JPEG"))
                    {
                        jpegICI = arrayICI[x];//设置JPEG编码
                        break;
                    }
                }

                if (jpegICI != null)
                {
                    outBmp.Save(destFile, jpegICI, encoderParams);
                }
                else
                {
                    outBmp.Save(destFile, thisFormat);
                }

                return true;
            }
            catch
            {
                return false;
            }
            finally
            {
                imgSource.Dispose();
                outBmp.Dispose();
            }
        }
}
}