Graphics.DrawString() 生成的图片质量比原图片差很多
System.Drawing.Bitmap addBitmap = new Bitmap(StringFileName);

  Graphics G = Graphics.FromImage(addBitmap);

  G.DrawImage(addBitmap, 0, 0, addBitmap.Width, addBitmap.Height);


  G.InterpolationMode = System.Drawing.Drawing2D.InterpolationMode.HighQualityBicubic;

  G.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.HighQuality;

  G.TextRenderingHint = System.Drawing.Text.TextRenderingHint.ClearTypeGridFit;

  G.CompositingQuality = System.Drawing.Drawing2D.CompositingQuality.HighQuality;
  G.PixelOffsetMode = System.Drawing.Drawing2D.PixelOffsetMode.HighQuality; 



  G.DrawString(StringSYName, richTextBox1.Font, new SolidBrush(richTextBox1.ForeColor), Left, Top);

  G.Dispose();


addBitmap.Save("文件名")  

源文件的大小有110多KB，很清楚的一张图片，加文字水印后，保存后的新文件，只有30K的样子，有点模糊，文件质量大不如以前，
请问有什么办法让加水印后的图片和源图片一样清晰，效果一样的不错，图片文件大小也和原来差不多！

------解决方案--------------------
保存的时候要设计图片质量的
 
Bitmap bitmap = new Bitmap(width, height);
//修改图片保存质量
ImageCodecInfo myImageCodecInfo = GetEncoderInfo("image/jpeg");
myEncoder = Encoder.Quality;
myEncoderParameters = new EncoderParameters(1);
//图片质量等级
myEncoderParameter = new EncoderParameter(myEncoder, 100L);
myEncoderParameters.Param[0] = myEncoderParameter;
//建立做图纸
Graphics g = Graphics.FromImage(bitmap);
g.Clear(Color.White);
//做图质量
g.SmoothingMode = SmoothingMode.AntiAlias;

//图像输出
bitmap.Save(Response.OutputStream, myImageCodecInfo, myEncoderParameters);