我做了一个视频上传的页面，可以截取视频的第一帧，发现好多视频截的图都是一样的，我想截取大约7分钟后的图片应该怎样修改
PublicMethod类如下：
using System;
using System.Configuration;
/// <summary>
/// Summary description for PublicMethod
/// </summary>
public class PublicMethod:System.Web.UI.Page
{
public PublicMethod()
{
}
//文件路径
public static string ffmpegtool = ConfigurationManager.AppSettings["ffmpeg"];
public static string mencodertool = ConfigurationManager.AppSettings["mencoder"];
public static string mplayertool = ConfigurationManager.AppSettings["mplayer"];
public static string upFile = ConfigurationManager.AppSettings["upfile"] + "/";
public static string imgFile = ConfigurationManager.AppSettings["imgfile"] + "/";
public static string playFile = ConfigurationManager.AppSettings["playfile"] + "/";
//文件图片大小
public static string sizeOfImg = ConfigurationManager.AppSettings["CatchFlvImgSize"];
//文件大小
public static string widthOfFile = ConfigurationManager.AppSettings["widthSize"];
public static string heightOfFile = ConfigurationManager.AppSettings["heightSize"];
// // //获取文件的名字
public static string GetFileName(string fileName)
{
int i = fileName.LastIndexOf("\\") + 1;
string Name = fileName.Substring(i);
return Name;
}
//获取文件扩展名
public static string GetExtension(string fileName)
{
int i = fileName.LastIndexOf(".")+1;
string Name = fileName.Substring(i);
return Name;
}
//
#region //运行FFMpeg的视频解码，(这里是绝对路径)
/// <summary>
/// 转换文件并保存在指定文件夹下面(这里是绝对路径)
/// </summary>
/// <param name="fileName">上传视频文件的路径（原文件）</param>
/// <param name="playFile">转换后的文件的路径（网络播放文件）</param>
/// <param name="imgFile">从视频文件中抓取的图片路径</param>
/// <returns>成功:返回图片虚拟地址; 失败:返回空字符串</returns>
public string ChangeFilePhy(string fileName, string playFile, string imgFile)
{
//取得ffmpeg.exe的路径,路径配置在Web.Config中,
//如:<add key="ffmpeg" value="E:\51aspx\ffmpeg.exe" />
string ffmpeg = Server.MapPath(PublicMethod.ffmpegtool);
if ((!System.IO.File.Exists(ffmpeg)) || (!System.IO.File.Exists(fileName))) {