在C#中將.bmp轉(zhuǎn)為.jpg格式的函數(shù)或類是什么，以及jpg文件的壓縮函數(shù)
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自己寫個(gè)，
 public class Bmp2Jpeg
 {
  private long lQuality=50L;
  private ImageCodecInfo myImageCodecInfo;
  private EncoderParameters myEncoderParameters;
  public Bmp2Jpeg(long Quality)
  {
   lQuality=Quality;
   Ini();
  }
  private void Ini()
  {
   System.Drawing.Imaging.Encoder myEncoder;
   EncoderParameter myEncoderParameter;
   myImageCodecInfo = GetEncoderInfo("image/jpeg");
   myEncoder = System.Drawing.Imaging.Encoder.Quality;
   myEncoderParameters = new EncoderParameters(1);
   myEncoderParameter = new EncoderParameter(myEncoder,lQuality);
   myEncoderParameters.Param[0] = myEncoderParameter;
  }
  public Bmp2Jpeg()
  {
   Ini();
  }
  private static ImageCodecInfo GetEncoderInfo(String mimeType)
  {
   int j;
   ImageCodecInfo[] encoders;
   encoders = ImageCodecInfo.GetImageEncoders();
   for(j = 0; j < encoders.Length; ++j)
   {
    if(encoders[j].MimeType == mimeType)
     return encoders[j];
   }
   return null;
  }
  public byte[] BmpToJpegBuff(Bitmap bmpSrc)
  {
   MemoryStream msjpg=new MemoryStream();
   bmpSrc.Save(msjpg, myImageCodecInfo, myEncoderParameters);
   byte[] bjpeg=msjpg.ToArray();
   
   return bjpeg;

  }
  public byte[] BmpToJpegBuff(byte[] bmpSrc)
  {
   MemoryStream ms=new MemoryStream(bmpSrc);
   MemoryStream msjpg=new MemoryStream();
   Bitmap myBitmap = new Bitmap(ms,false);
   //myBitmap.Save("c://temp//"+i.ToString()+".jpg", myImageCodecInfo, myEncoderParameters);
   myBitmap.Save(msjpg, myImageCodecInfo, myEncoderParameters);
   byte[] bjpeg=msjpg.ToArray();
   
   return bjpeg;
  }
  public Stream BmpToJpegStream(byte[] bmpSrc)
  {
   MemoryStream ms=new MemoryStream(bmpSrc);
   MemoryStream msjpg=new MemoryStream();
   Bitmap myBitmap = new Bitmap(ms,false);
   //myBitmap.Save("c://temp//"+i.ToString()+".jpg", myImageCodecInfo, myEncoderParameters);
   myBitmap.Save(msjpg, myImageCodecInfo, myEncoderParameters);
   return msjpg;
  }
  public bool BmpToJpegFile(byte[] bmpSrc,string filename)
  {
   MemoryStream ms=new MemoryStream(bmpSrc);
   Bitmap myBitmap=new Bitmap(ms,false);
   myBitmap.Save(filename,myImageCodecInfo,myEncoderParameters);
   return true;
  }
  public bool BmpToJpegFile1(Bitmap bmpSrc,string filename)
  {
   bmpSrc.Save(filename,myImageCodecInfo,myEncoderParameters);
   return true;
  }
 }

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最簡(jiǎn)單的方式應(yīng)該是：
Bitmap image = new Bitmap("Your image path.bmp");
image.Save("new jpg file path", ImageFormat.Jpg);

注意：別忘了引入名稱空間。

另外，jpg的壓縮率相對(duì)于相同內(nèi)容大小精度的Bmp來說也是不同的，總之很大就是了。尤其是簡(jiǎn)單的圖形壓縮率很好。
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ImageFormat 類（在命名空間 System.Drawing.Imaging 下）,就可以實(shí)現(xiàn)BMP、GIF、Icon、JPEG、PNG、TIFF、WMF等常見圖象格式之間的轉(zhuǎn)化了。
打開需要轉(zhuǎn)換的文件，存放在一個(gè)變量中
bitmap.Save(fileName, ImageFormat.Jpg);