根据左上角坐标、长、高，实现截图

        /// <summary>
        /// 根据左上角坐标、长、高，实现截图
        /// </summary>
        /// <param name="vehicleImage">图片二进制流</param>
        /// <param name="image1Url">图片路径</param>
        /// <param name="vehicleRegionLeft">左上角X轴坐标</param>
        /// <param name="vehicleRegionTop">左上角Y轴坐标</param>
        /// <param name="vehicleRegionWidth">宽度</param>
        /// <param name="vehicleRegionHeight">高度</param>
        /// <returns></returns>
        public ActionResult GetVehicleImageByRegion(string vehicleImage, string image1Url, 
            int vehicleRegionLeft, int vehicleRegionTop, int vehicleRegionWidth, int vehicleRegionHeight)
        {
            if (!string.IsNullOrEmpty(vehicleImage) && FileHelper.Exists(vehicleImage))
            {
                byte[] buffer = FileHelper.ReadAllBytes(vehicleImage);
                return File(buffer, "image/jpeg");
            }
            else if (!string.IsNullOrEmpty(image1Url))
            {
                Bitmap image1Bitmap = null;
                Bitmap newBit = null;

                if (image1Url.ToLower().Contains("http")) //http路由
                {
                    WebRequest request = WebRequest.Create(image1Url);
                    WebResponse response = request.GetResponse();

                    if(response == null)
                        return File(new byte[0], "image/jpeg");

                    Stream stream = response.GetResponseStream();
                    image1Bitmap = (Bitmap)Image.FromStream(stream);
                }

                else //ftp路径
                {
                    if (!FileHelper.Exists(image1Url))
                        return File(new byte[0], "image/jpeg");

                    image1Bitmap = (Bitmap)Image.FromFile(image1Url);
                }

                try
                {
                    newBit = new Bitmap(vehicleRegionWidth, vehicleRegionHeight);

                    using (Graphics g = Graphics.FromImage(newBit))  //实现截图
                    {
                        g.DrawImage(image1Bitmap,
                            new Rectangle(0, 0, newBit.Width, newBit.Height),
                            new Rectangle(vehicleRegionLeft, vehicleRegionTop,
                                newBit.Width, newBit.Height),
                            GraphicsUnit.Pixel);

                        using (MemoryStream ms = new MemoryStream())
                        {
                            newBit.Save(ms, ImageFormat.Jpeg);  //保存图片
                            return File(ms.ToArray(), "image/jpeg");
                        }
                    }
                }
                finally
                {
                    if (image1Bitmap != null)
                        image1Bitmap.Dispose();

                    if (newBit != null)
                        newBit.Dispose();
                }
            }


            return File(new byte[0], "image/jpeg");
        }