How to extract the inside and outside contours of an image. Why my code make sense ,but doesn't wrork well.

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I have been working on my dissertation lately .Now I get a tough problem to deal with .When I call
the following function to do contour tracking ,I found there are some points cannot be tracked. I use the setpixel function to set the points that has been tracked in white color, but the contour doesn't disappeared as I expected,there are still some points left which make it difficult to track the next contour. Help!

Code:

public void  MyFindContours(Bitmap cBmp)
{ 
   int[,] Direction = new int[,] { { -1, 1 }, { 0, 1 }, { 1, 1 }, { 1, 0 }, { 1, -1 }, { 0, -1 }, { -1, -1 }, { -1, 0 } };

   Color PointPix = new Color();
   Point StartPoint = new Point();
   Point CurrentPoint = new Point();

   bool bFindStartPoint = false;
   bool bFindPoint;

   Color bb = Color.FromArgb(0, 0, 0);
   Color ww = Color.FromArgb(255, 255, 255);

   //先找最左上方的边界点
   for (int j = 0; j < cBmp.Height && !bFindStartPoint; j++)
   {
      for (int i = 0; i < cBmp.Width && !bFindStartPoint; i++)
      {
         PointPix = cBmp.GetPixel(i, j);

         if (PointPix == bb)
         {
            bFindStartPoint = true;
            StartPoint.X = i;
            StartPoint.Y = j;
         
            myContour.Clear(); //每次找到一个轮廓边界点,将myContour 清空，用于暂时存放轮廓点
            myContour.Add(StartPoint);//将轮廓起点存入
         }
         else
         {
            if (j == cBmp.Height - 1 && i == cBmp.Width - 1) 
            {
               return;
            }
            else
            {
               continue;
            }
         }
      }
   }

   //由于起始点在左下方，故起始扫描沿着左上方向

   int BeginDirect = 0;
   bFindStartPoint = false;

   CurrentPoint.X = StartPoint.X;
   CurrentPoint.Y = StartPoint.Y;

   while (!bFindStartPoint)
   {
      bFindPoint = false;

      while (!bFindPoint)
      {
         PointPix = cBmp.GetPixel(CurrentPoint.X + Direction[BeginDirect, 0], CurrentPoint.Y + Direction[BeginDirect, 1]);

         if (PointPix == bb)
         {
            bFindPoint = true;

            myContour.Add(CurrentPoint);//当前点为边界点，则将其加入数组

            cBmp.SetPixel(CurrentPoint.X,CurrentPoint.Y, ww);
            this.pictureBox2.Image = cBmp;
            this.pictureBox2.Refresh();

            CurrentPoint.Y = CurrentPoint.Y + Direction[BeginDirect, 1];
            CurrentPoint.X = CurrentPoint.X + Direction[BeginDirect, 0];

            if (CurrentPoint.X == StartPoint.X && CurrentPoint.Y == StartPoint.Y)
            {
               bFindStartPoint = true;
               myContour.Remove(CurrentPoint);//当回到起点，已经把起点加到数组最后了，因此要删除
               gContour.Add(myContour);//把轮廓点加到总轮廓中
            }

            PointPix = cBmp.GetPixel(CurrentPoint.X,CurrentPoint.Y);
            //扫描的方向逆时针旋转两格
            BeginDirect--;

            if (BeginDirect == -1)
            {
               BeginDirect = 7;
            }

            BeginDirect--;

            if (BeginDirect == -1)
            {
               BeginDirect = 7;
            }
         }
         else
         {
            //扫描方向顺时针旋转一格
            BeginDirect++;

            if (BeginDirect == 8)
            {
               BeginDirect = 0;
            }
         }
      }
   }

   for(int i=0;i<gcontour.count;i++)>
   {
      for (int j = 0; j < gContour[i].Count; j++)
      {
         cBmp.SetPixel(gContour[i].ElementAt(j).X, gContour[i].ElementAt(j).Y, ww);
         this.pictureBox2.Image = cBmp;
         this.pictureBox2.Refresh();
      }
   }
        
   MyFindContours(cBmp);
}