C# 大图片压缩算法，减少图片体积

private void button3_Click(object sender, EventArgs e)
        {
            using (OpenFileDialog ofd = new OpenFileDialog())
            {
                ofd.Filter = "图片|*.jpg;*.png;*.gif;*.jpeg;*.bmp";
                if (ofd.ShowDialog() == DialogResult.OK)
                {
                    using (FileStream file = new FileStream(ofd.FileName, FileMode.Open))
                    {
                        var img = CompressionImage(file, 50L);
                        MessageBox.Show(CreateImageFromBytes(AppDomain.CurrentDomain.BaseDirectory + @"\" + Guid.NewGuid().ToString(), img));
                    }

                }
            }
        }

        /// <summary>
        /// Convert Byte[] to Image
        /// </summary>
        /// <param name="buffer"></param>
        /// <returns></returns>
        public static Image BytesToImage(byte[] buffer)
        {
            MemoryStream ms = new MemoryStream(buffer);
            Image image = System.Drawing.Image.FromStream(ms);
            return image;
        }

        /// <summary>
        /// Convert Byte[] to a picture and Store it in file
        /// </summary>
        /// <param name="fileName"></param>
        /// <param name="buffer"></param>
        /// <returns></returns>
        public static string CreateImageFromBytes(string fileName, byte[] buffer)
        {
            string file = fileName;
            Image image = BytesToImage(buffer);
            ImageFormat format = image.RawFormat;
            if (format.Equals(ImageFormat.Jpeg))
            {
                file += ".jpeg";
            }
            else if (format.Equals(ImageFormat.Png))
            {
                file += ".png";
            }
            else if (format.Equals(ImageFormat.Bmp))
            {
                file += ".bmp";
            }
            else if (format.Equals(ImageFormat.Gif))
            {
                file += ".gif";
            }
            else if (format.Equals(ImageFormat.Icon))
            {
                file += ".icon";
            }
            System.IO.FileInfo info = new System.IO.FileInfo(file);
            System.IO.Directory.CreateDirectory(info.Directory.FullName);
            File.WriteAllBytes(file, buffer);
            return file;
        }
        /// <summary>
        /// 壓縮圖片 /// </summary>
        /// <param name="fileStream">圖片流</param>
        /// <param name="quality">壓縮質量0-100之間 數值越大質量越高</param>
        /// <returns></returns>
        private byte[] CompressionImage(Stream fileStream, long quality)
        {
            using (System.Drawing.Image img = System.Drawing.Image.FromStream(fileStream))
            {
                using (Bitmap bitmap = new Bitmap(img))
                {
                    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;
        }