压缩png质量不改变像素

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private static  byte[] CompressionImage(Bitmap bitmap, Stream fileStream, long quality)     {         using (System.Drawing.Image img = System.Drawing.Image.FromStream(fileStream))         {               ImageCodecInfo CodecInfo = GetEncoder(img.RawFormat);             System.Drawing.Imaging.Encoder myEncoder = System.Drawing.Imaging.Encoder.Quality;             EncoderParameters myEncoderParameters = new EncoderParameters(1);             EncoderParameter myEncoderParameter = new EncoderParameter(myEncoder, quality);             myEncoderParameters.Param[0] = myEncoderParameter;             using (MemoryStream ms = new MemoryStream())             {                                  bitmap.Save(ms, CodecInfo, myEncoderParameters);                 myEncoderParameters.Dispose();                 myEncoderParameter.Dispose();                 return ms.ToArray();             }           }     }         private static ImageCodecInfo GetEncoder(ImageFormat format)     {         ImageCodecInfo[] codecs = ImageCodecInfo.GetImageDecoders();           foreach (ImageCodecInfo codec in codecs)         {             if (codec.FormatID == format.Guid)             {                 return codec;             }         }         return null;     }