#region GetPicThumbnail
        /// <summary>
        /// 无损压缩图片
        /// </summary>
        /// <param name="sm">文件流</param>
        /// <param name="flag">压缩质量 1-100</param>
        /// <returns></returns>
        public static byte[] GetPicThumbnail(Stream sm, int flag)
        {
            Image iSource = Image.FromStream(sm);
            //获取图片 EXIF信息  如果图片旋转了 在给转回来
            var dic = ReadExif(iSource);
            if (dic.Count > 0 && dic["Orientation"] == "6")
            {
                iSource.RotateFlip(RotateFlipType.Rotate90FlipX);
            }
            ImageFormat tFormat = iSource.RawFormat;
            int sW, sH;
            //按比例缩放
            Size temsize = new Size(iSource.Width, iSource.Height);
            int dWidth = temsize.Width;
            int dHeight = temsize.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(0, 0, iSource.Width, iSource.Height), 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(System.Drawing.Imaging.Encoder.Quality, qy);
            ep.Param[0] = eParam;
            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;
                    }
                }
                byte[] bytes;
                if (jpegICIinfo != null)
                {
                    MemoryStream ms = new MemoryStream();
                    ob.Save(ms, jpegICIinfo, ep);//dFile是压缩后的新路径
                    bytes = ms.GetBuffer();  //byte[]   bytes=   ms.ToArray(); 这两句都可以，至于区别么，下面有解释
                    ms.Close();
                }
                else
                {
                    MemoryStream ms = new MemoryStream();
                    ob.Save(ms, tFormat);
                    bytes = ms.GetBuffer();  //byte[]   bytes=   ms.ToArray(); 这两句都可以，至于区别么，下面有解释
                    ms.Close();
                }
                return bytes;
            }
            catch
            {
                return null;
            }
            finally
            {
                iSource.Dispose();
                ob.Dispose();
            }
        }
        /// <summary>
        /// 获取图片EXIF信息
        /// </summary>
        /// <param name="image"></param>
        /// <returns></returns>
        private static Dictionary<string, string> ReadExif(Image image)
        {
            var exif = new Dictionary<string, string>();
            var properties = image.PropertyItems;
            foreach (var property in properties)
            {
                switch (property.Id)
                {
                    case 0x010E:
                        exif["ImageTitle"] = ASCIIToString(property.Value);
                        break;
                    case 0x010F:
                        exif["Make"] = ASCIIToString(property.Value);
                        break;
                    case 0x0110:
                        exif["Model"] = ASCIIToString(property.Value);
                        break;
                    case 0x0112:
                        exif["Orientation"] = ShortToString(property.Value, 0);
                        break;
                    case 0x011A:
                        exif["XResolution"] = RationalToSingle(property.Value, 0);
                        break;
                    case 0x011B:
                        exif["YResolution"] = RationalToSingle(property.Value, 0);
                        break;
                    case 0x0128:
                        exif["ResolutionUnit"] = ShortToString(property.Value, 0);
                        break;
                    case 0x0131:
                        exif["Software"] = ASCIIToString(property.Value);
                        break;
                    case 0x0132:
                        exif["DateTime"] = ASCIIToString(property.Value);
                        break;
                    //GPS
                    case 0x0002:
                        exif["GPSLatitude"] = string.Format("{0}°{1}′{2}″",
                                                            RationalToSingle(property.Value, 0),
                                                            RationalToSingle(property.Value, 8),
                                                            RationalToSingle(property.Value, 16)
                                                            );
                        break;
                    case 0x0004:
                        exif["GPSLongitude"] = string.Format("{0}°{1}′{2}″",
                                                            RationalToSingle(property.Value, 0),
                                                            RationalToSingle(property.Value, 8),
                                                            RationalToSingle(property.Value, 16)
                                                            );
                        break;
                    case 0x0006:
                        exif["GPSAltitude"] = RationalToSingle(property.Value, 0);
                        break;
                }
            }
            return exif;
        }
        #region tostring方法
        static string ByteToString(byte[] b, int startindex)
        {
            if (startindex + 1 <= b.Length)
                return ((char)b[startindex]).ToString();
            else
                return string.Empty;
        }
        static string ShortToString(byte[] b, int startindex)
        {
            if (startindex + 2 <= b.Length)
                return BitConverter.ToInt16(b, startindex).ToString();
            else
                return string.Empty;
        }
        static string RationalToSingle(byte[] b, int startindex)
        {
            if (startindex + 8 <= b.Length)
                return (BitConverter.ToSingle(b, startindex) / BitConverter.ToSingle(b, startindex + 4)).ToString();
            else
                return string.Empty;
        }
        static string ASCIIToString(byte[] b)
        {
            return Encoding.ASCII.GetString(b);
        }
        #endregion
        #endregion