文件上传，服务端压缩文件方法，重点是png与gif图片的压缩，保证了透明度与动画

/// <summary>
    /// 上传文件帮助类
    /// </summary>
    public class ImageUploadHelper
    {

        #region SaveVideoFirstImg 根据视频路径生成视频对应的图片
        /// <summary>
        /// 根据视频路径生成视频对应的图片
        /// </summary>
        /// <param name="videoUrl">视频的绝对路径</param>
        public static void SaveVideoFirstImg(string videoUrl)
        {
            var saveFileurl = videoUrl.Substring(0, videoUrl.LastIndexOf(".")) + ".jpg";
            Process p = new Process();
            p.StartInfo.FileName = string.Format("{0}\ffmpeg.exe", HttpContext.Current.Server.MapPath("~/media/"));
            p.StartInfo.CreateNoWindow = true;
            p.StartInfo.UseShellExecute = false;
            p.StartInfo.Arguments = string.Format("-i {0} -y -f image2 -t 1 {1}", videoUrl, saveFileurl);
            p.Start();
            p.WaitForExit();
            p.Close();
            p.Dispose();
        }
        #endregion

        #region CompressImg 生成缩略图方法
        /// <summary>
        /// 生成缩略图方法
        /// </summary>
        /// <param name="oldUrl">完整原始图片路径</param>
        /// <param name="newUrl">完整缩略图路径</param>
        /// <param name="thumbWidth">缩略图宽度</param>
        /// <param name="thumbHeight">缩略图高度</param>
        /// <returns>成功生成返回true，失败生成返回false</returns>
        public static bool CompressImg(string oldUrl, string newUrl, double thumbWidth, double thumbHeight)
        {
            if (File.Exists(oldUrl))
            {
                //原图加载
                using (System.Drawing.Image sourceImage = System.Drawing.Image.FromFile(oldUrl))
                {
                    var imgFormate = GetImageFormat(sourceImage);

                    //原图宽高均小于模版，不作处理，直接保存
                    if (sourceImage.Width <= thumbWidth && sourceImage.Height <= thumbHeight)
                    {
                        //保存
                        sourceImage.Save(newUrl, imgFormate);
                    }
                    else
                    {
                        //缩略图宽、高计算
                        double newWidth = sourceImage.Width;
                        double newHeight = sourceImage.Height;

                        //宽大于高或宽等于高（横图或正方）
                        if (sourceImage.Width >= sourceImage.Height) 
                        {
                            //如果宽大于模版
                            if (sourceImage.Width > thumbWidth)
                            {
                                //宽按模版，高按比例缩放
                                newWidth = thumbWidth;
                                newHeight = sourceImage.Height * (thumbWidth / sourceImage.Width);
                            }
                        }
                        //高大于宽（竖图）
                        else
                        {
                            //如果高大于模版
                            if (sourceImage.Height > thumbHeight)
                            {
                                //高按模版，宽按比例缩放
                                newHeight = thumbHeight;
                                newWidth = sourceImage.Width * (thumbHeight / sourceImage.Height);
                            }
                        }

                        //判断是否为gif或者tiff图片
                        if (imgFormate == ImageFormat.Gif || imgFormate == ImageFormat.Tiff)
                        {
                            //压缩图片
                            CompressImgGif(sourceImage, imgFormate, newUrl, (int)newWidth, (int)newHeight);
                        }
                        else //其它格式图片
                        {
                            //生成新图,新建一个bmp图片
                            System.Drawing.Image newImage = new System.Drawing.Bitmap((int)newWidth, (int)newHeight);

                            //新建一个画板
                            System.Drawing.Graphics newG = System.Drawing.Graphics.FromImage(newImage);

                            //置背景色
                            newG.Clear(Color.Transparent);

                            //设置质量
                            newG.InterpolationMode = InterpolationMode.HighQualityBicubic;
                            newG.SmoothingMode = SmoothingMode.HighQuality;

                            //画图
                            newG.DrawImage(sourceImage, new System.Drawing.Rectangle(0, 0, newImage.Width, newImage.Height), new System.Drawing.Rectangle(0, 0, sourceImage.Width, sourceImage.Height), System.Drawing.GraphicsUnit.Pixel);

                            newImage.Save(newUrl, imgFormate);

                            //释放资源
                            newG.Dispose();
                            newImage.Dispose();
                        }
                    }
                }
                return true;
            }

            return false;
        }
        #endregion

        #region CompressImgGif 压缩gif图片
        /// <summary>
        /// 压缩gif图片
        /// </summary>
        /// <param name="oldUrl"></param>
        /// <param name="newUrl"></param>
        /// <param name="thumbWidth"></param>
        /// <param name="thumbHeight"></param>
        public static void CompressImgGif(Image sourceImage, ImageFormat format, string newUrl, int width, int height)
        {
            //新图第一帧
            Image new_img = new Bitmap(width, height);

            //新图其他帧
            Image new_imgs = new Bitmap(width, height);

            //新图第一帧GDI+绘图对象
            Graphics g_new_img = Graphics.FromImage(new_img);

            //新图其他帧GDI+绘图对象
            Graphics g_new_imgs = Graphics.FromImage(new_imgs);

            //配置新图第一帧GDI+绘图对象
            g_new_img.CompositingMode = CompositingMode.SourceCopy;
            g_new_img.InterpolationMode = InterpolationMode.HighQualityBicubic;
            g_new_img.PixelOffsetMode = PixelOffsetMode.HighQuality;
            g_new_img.SmoothingMode = SmoothingMode.HighQuality;
            g_new_img.Clear(Color.FromKnownColor(KnownColor.Transparent));

            //配置其他帧GDI+绘图对象
            g_new_imgs.CompositingMode = CompositingMode.SourceCopy;
            g_new_imgs.InterpolationMode = InterpolationMode.HighQualityBicubic;
            g_new_imgs.PixelOffsetMode = PixelOffsetMode.HighQuality;
            g_new_imgs.SmoothingMode = SmoothingMode.HighQuality;
            g_new_imgs.Clear(Color.FromKnownColor(KnownColor.Transparent));

            //遍历维数
            foreach (Guid gid in sourceImage.FrameDimensionsList)
            {
                //因为是缩小GIF文件所以这里要设置为Time
                //如果是TIFF这里要设置为PAGE
                FrameDimension f = format == ImageFormat.Gif ? FrameDimension.Time : FrameDimension.Page;

                //获取总帧数
                int count = sourceImage.GetFrameCount(f);
                //保存标示参数
                System.Drawing.Imaging.Encoder encoder = System.Drawing.Imaging.Encoder.SaveFlag;
                //
                EncoderParameters ep = null;

                //每一帧
                for (int c = 0; c < count; c++)
                {
                    //选择由维度和索引指定的帧
                    sourceImage.SelectActiveFrame(f, c);
                    //第一帧
                    if (c == 0)
                    {
                        //将原图第一帧画给新图第一帧
                        g_new_img.DrawImage(sourceImage, new Rectangle(0, 0, new_img.Width,new_img.Height), new Rectangle(0, 0, sourceImage.Width, sourceImage.Height), GraphicsUnit.Pixel);

                        //把振频和透明背景调色板等设置复制给新图第一帧
                        for (int i = 0; i < sourceImage.PropertyItems.Length; i++)
                        {
                            new_img.SetPropertyItem(sourceImage.PropertyItems[i]);
                        }
                        ep = new EncoderParameters(1);
                        //第一帧需要设置为MultiFrame
                        ep.Param[0] = new EncoderParameter(encoder, (long)EncoderValue.MultiFrame);

                        //保存第一帧
                        new_img.Save(newUrl, GetCodecInfo(format.Guid), ep);
                    }
                    //其他帧
                    else
                    {
                        //把原图的其他帧画给新图的其他帧
                        g_new_imgs.DrawImage(sourceImage, new Rectangle(0, 0, new_imgs.Width,new_imgs.Height), new Rectangle(0, 0, sourceImage.Width, sourceImage.Height), GraphicsUnit.Pixel);

                        //把振频和透明背景调色板等设置复制给新图第一帧
                        for (int i = 0; i < sourceImage.PropertyItems.Length; i++)
                        {
                            new_imgs.SetPropertyItem(sourceImage.PropertyItems[i]);
                        }
                        ep = new EncoderParameters(1);

                        //如果是GIF这里设置为FrameDimensionTime
                        //如果为TIFF则设置为FrameDimensionPage
                        ep.Param[0] = new EncoderParameter(encoder, (long)(format == ImageFormat.Gif ? EncoderValue.FrameDimensionTime : EncoderValue.FrameDimensionPage));

                        //向新图添加一帧
                        new_img.SaveAdd(new_imgs, ep);
                    }
                }

                ep = new EncoderParameters(1);
                //关闭多帧文件流
                ep.Param[0] = new EncoderParameter(encoder, (long)EncoderValue.Flush);
                new_img.SaveAdd(ep);
            }

            //释放文件
            new_img.Dispose();
            new_imgs.Dispose();
            g_new_img.Dispose();
            g_new_imgs.Dispose();
        }
        #endregion

        #region 压缩文件
        /// <summary>
        /// 压缩文件
        /// </summary>
        /// <param name="oldImg"></param>
        /// <param name="newImg"></param>
        public static void CompressSize(string oldImg, string newImg)
        {

            using (System.Drawing.Image sourceImage = System.Drawing.Image.FromFile(oldImg))
            {
                Bitmap bmp = new Bitmap(sourceImage);
                KiSaveAsJPEG(bmp, newImg, 20);
            }

        }

        //// <summary>
        /// 保存为JPEG格式，支持压缩质量选项
        /// </summary>
        /// <param name="bmp"></param>
        /// <param name="FileName"></param>
        /// <param name="Qty"></param>
        /// <returns></returns>
        public static bool KiSaveAsJPEG(Bitmap bmp, string FileName, int Qty)
        {
            try
            {
                EncoderParameter p;
                EncoderParameters ps;

                ps = new EncoderParameters(1);

                p = new EncoderParameter(System.Drawing.Imaging.Encoder.Quality, Qty);
                ps.Param[0] = p;

                bmp.Save(FileName, GetCodecInfo("image/jpeg"), ps);

                return true;
            }
            catch
            {
                return false;
            }
        }

        //// <summary>
        /// 获取codeinfo
        /// </summary>
        /// <param name="mimeType"></param>
        /// <returns>得到指定mimeType的ImageCodecInfo</returns>
        private static ImageCodecInfo GetCodecInfo(string mimeType)
        {
            ImageCodecInfo[] CodecInfo = ImageCodecInfo.GetImageEncoders();
            foreach (ImageCodecInfo ici in CodecInfo)
            {
                if (ici.MimeType == mimeType) return ici;
            }
            return null;
        }

        /// <summary>
        /// 获取codeinfo
        /// </summary>
        /// <param name="guid"></param>
        /// <returns></returns>
        public static ImageCodecInfo GetCodecInfo(Guid guid)
        {
            ImageCodecInfo[] icis = ImageCodecInfo.GetImageDecoders();

            //为 图片编码、解码器 对象 赋值
            foreach (ImageCodecInfo ici in icis)
            {
                if (ici.FormatID == guid)
                {
                    return ici;
                }
            }

            return null;
        }
        #endregion

        #region GetImageFormat 获取图片文件格式
        /// <summary>
        /// 获取图片文件格式
        /// </summary>
        /// <param name="img"></param>
        /// <returns></returns>
        public static ImageFormat GetImageFormat(Image img)
        {
            if (img != null)
            {
                ImageFormat fileExtension = img.RawFormat,
                       jpgExtension = ImageFormat.Jpeg,
                       gifExtension = ImageFormat.Gif,
                       pngExtension = ImageFormat.Png,
                       iconExtension = ImageFormat.Icon,
                       bmpExtension = ImageFormat.Bmp,
                       tiffExtension = ImageFormat.Tiff,
                       wmfExtension = ImageFormat.Wmf,
                       emfExtension = ImageFormat.Emf,
                       exifExtension = ImageFormat.Exif;

                if (fileExtension.Equals(jpgExtension))
                {
                    return ImageFormat.Jpeg;
                }
                else if (fileExtension.Equals(gifExtension))
                {
                    return ImageFormat.Gif;
                }
                else if (fileExtension.Equals(pngExtension))
                {
                    return ImageFormat.Png;
                }
                else if (fileExtension.Equals(iconExtension))
                {
                    return ImageFormat.Icon;
                }
                else if (fileExtension.Equals(bmpExtension))
                {
                    return ImageFormat.Bmp;
                }
                else if (fileExtension.Equals(tiffExtension))
                {
                    return ImageFormat.Tiff;
                }
                else if (fileExtension.Equals(wmfExtension))
                {
                    return ImageFormat.Wmf;
                }
                else if (fileExtension.Equals(emfExtension))
                {
                    return ImageFormat.Emf;
                }
                else if (fileExtension.Equals(exifExtension))
                {
                    return ImageFormat.Exif;
                }
            }
            return ImageFormat.Jpeg;
        }
        #endregion

        #region GetExtension 获得上传文件的扩展名
        /// <summary>
        /// 获得上传文件的扩展名
        /// </summary>
        /// <param name="img"></param>
        /// <returns></returns>
        public static string GetExtension(Image img)
        {
            if (img != null)
            {
                ImageFormat fileExtension = img.RawFormat,
                      jpgExtension = ImageFormat.Jpeg,
                      gifExtension = ImageFormat.Gif,
                      pngExtension = ImageFormat.Png,
                      iconExtension = ImageFormat.Icon,
                      bmpExtension = ImageFormat.Bmp,
                      tiffExtension = ImageFormat.Tiff,
                      wmfExtension = ImageFormat.Wmf,
                      emfExtension = ImageFormat.Emf,
                      exifExtension = ImageFormat.Exif;

                if (fileExtension.Equals(jpgExtension))
                {
                    return ".jpg";
                }
                else if (fileExtension.Equals(gifExtension))
                {
                    return ".gif";
                }
                else if (fileExtension.Equals(pngExtension))
                {
                    return ".png";
                }
                else if (fileExtension.Equals(iconExtension))
                {
                    return ".icon";
                }
                else if (fileExtension.Equals(bmpExtension))
                {
                    return ".bmp";
                }
                else if (fileExtension.Equals(tiffExtension))
                {
                    return ".tiff";
                }
                else if (fileExtension.Equals(wmfExtension))
                {
                    return ".wmf";
                }
                else if (fileExtension.Equals(emfExtension))
                {
                    return ".emf";
                }
                else if (fileExtension.Equals(exifExtension))
                {
                    return ".exif";
                }
            }
            return ".jpg";
        }
        #endregion

    }

    #region UploadImageMsg 上传图片后返回上传结果
    /// <summary>
    /// 上传图片后返回上传结果
    /// </summary>
    public class UploadImageMsg
    {
        public UploadImageMsg()
        {
            error = 1;
            url = message = string.Empty;
        }

        /// <summary>
        /// 1错误 0成功
        /// </summary>
        public int error { get; set; }
        /// <summary>
        /// 错误信息
        /// </summary>
        public string message { get; set; }
        /// <summary>
        /// 上传成功的url路径
        /// </summary>
        public string url { get; set; }
    }
    #endregion