压缩png质量不改变像素

    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;
        }