c# 无损压缩图片,接口传过来的是字节

    /// <summary>
        /// 无损压缩图片
        /// </summary>

        /// <param name="content">base64字节流</param>
        /// <param name="flag">（数字越小压缩率越高）1-100</param>
        /// <param name="size">压缩后图片的最大大小</param>
        /// <param name="sfsc">是否是第一次调用</param>
        /// <returns></returns>
        public static string CompressImage(string content, int flag = 90, int size = 500, bool sfsc = true)
        {
            //如果是第一次调用，原始图像的大小小于要压缩的大小，则直接复制文件，并且返回true
            byte[] bytes = Convert.FromBase64String(content);
            if (sfsc == true && bytes.Length < size * 1024)
            {
                return content;
            }

            MemoryStream stream = new MemoryStream(bytes, 0, bytes.Length);
            stream.Write(bytes, 0, bytes.Length);
            Image iSource = Image.FromStream(stream);
            ImageFormat tFormat = iSource.RawFormat;
            int dHeight = iSource.Height / 2;
            int dWidth = iSource.Width / 2;
            //按比例缩放
            Size tem_size = new Size(iSource.Width, iSource.Height);
            int sH;
            int sW;
            if (tem_size.Width > dHeight || tem_size.Width > dWidth)
            {
                if ((tem_size.Width * dHeight) > (tem_size.Width * dWidth))
                {
                    sW = dWidth;
                    sH = (dWidth * tem_size.Height) / tem_size.Width;
                }
                else
                {
                    sH = dHeight;
                    sW = (tem_size.Width * dHeight) / tem_size.Height;
                }
            }
            else
            {
                sW = tem_size.Width;
                sH = tem_size.Height;
            }

            Bitmap ob = new Bitmap(dWidth, dHeight);
            Graphics g = Graphics.FromImage(ob);

            g.Clear(Color.WhiteSmoke);
            g.CompositingQuality =CompositingQuality.HighQuality;
            g.SmoothingMode = SmoothingMode.HighQuality;
            g.InterpolationMode =InterpolationMode.HighQualityBicubic;

            g.DrawImage(iSource, new Rectangle((dWidth - sW) / 2, (dHeight - sH) / 2, sW, sH), 0, 0, iSource.Width, iSource.Height, GraphicsUnit.Pixel);

            g.Dispose();

            //以下代码为保存图片时，设置压缩质量
            EncoderParameters ep = new EncoderParameters();
            long[] qy = new long[1];
            qy[0] = flag;//设置压缩的比例1-100
            EncoderParameter eParam = new EncoderParameter(Encoder.Quality, qy);
            ep.Param[0] = eParam;
            //保存在内存流
            MemoryStream ms = new MemoryStream();
            ms.Position = 0;
            string imagebase64 = "";
            try
            {
                ImageCodecInfo[] arrayICI = ImageCodecInfo.GetImageEncoders();
                ImageCodecInfo jpegICIinfo = null;
                for (int x = 0; x < arrayICI.Length; x++)
                {
                    if (arrayICI[x].FormatDescription.Equals("JPEG"))
                    {
                        jpegICIinfo = arrayICI[x];
                        break;
                    }
                }
                if (jpegICIinfo != null)
                {
                 
                    ob.Save(ms, jpegICIinfo, ep);
                    byte[] arr = new byte[ms.Length];
                    ms.Read(arr, 0, (int)ms.Length);
                    imagebase64 = Convert.ToBase64String(arr);
                    if (arr.Length > 1024 * size)
                    {
                        flag -= 10;
                        CompressImage(content, flag, size, false);
                    }
                }
                else
                {
                    ob.Save(ms, tFormat);
                    byte[] arr = new byte[ms.Length];
                    ms.Read(arr, 0, (int)ms.Length);
                    imagebase64 = Convert.ToBase64String(bytes);
                 
                }
                return imagebase64;
            }
            catch
            {
                return imagebase64;
            }
            finally
            {
                iSource.Dispose();
                ob.Dispose();
                ms.Close();
                stream.Close();
            }


        }

　　写的不咋地，但是能用了。。嘿