Line detection method

Hi Guys, I'm trying to do some project to detect um object in microscope. I'm using OpenCV for some time, but I dont underestand everything... Goal is: 1. with movable cursor find centre line of two rectangular object 2. find contour line in each quadrant 3. via start and end point find if rectangles are parallel 4. print out diff angle, draw contour lines 5. calculate rectangles middle line and print distance of lines in center 6. **ALL OF IT DO FAST (2-10 FPS)** I'm using Qt with c\c++ and openCV **This is my image after BW threshold**  **And this is my image after Canny**  I'm not able to find lines after Canny via HoughLineP, and I don't know why Can you suggest my why ? Here is my source code

if(mat_orig.empty() == false)

   {
      cvtColor(mat_orig,mat_orig,CV_RGB2GRAY);
      int cursor_x, top_limit, bot_limit;
      int w_second;
      int h_mid;
      int o_x, o_y;

      o_x = mat_orig.cols;
      o_y = mat_orig.rows;

      cursor_x    = mat_orig.cols/2;                          //Here should be number from mouse x coord
      top_limit   = o_y/2 - o_y*0.05;
      bot_limit   = o_y/2 + o_y*0.05;

      w_second    = o_x - cursor_x;
      h_mid       = o_y - (2 * top_limit);

      //Make Partition
      mat_part[0]     = mat_orig(Rect(0,                0, cursor_x, top_limit));
      mat_part[1]     = mat_orig(Rect(cursor_x,         0, w_second, top_limit));
      mat_part[2]     = mat_orig(Rect(0,        bot_limit, cursor_x, top_limit));
      mat_part[3]     = mat_orig(Rect(cursor_x, bot_limit, w_second, top_limit));
      mat_part_mid    = mat_orig(Rect(0,        top_limit,      o_x,     h_mid));

      // some vars for process 
      int i=0;
      vector lines;
      Vec4i l;
      int dy_max = 1;
      char points[50];

      //loop calculate thresh, canny and hough for all 4 quads
      for (i=0; i<4; i++)
      {
         threshold(mat_part[i], mat_part[i], 0, 255, CV_THRESH_BINARY + CV_THRESH_OTSU);
         Canny(mat_part[i], mat_part[i], 200, 600, 3,true);
         HoughLinesP(mat_part[i], lines, 1, CV_PI/(180), 300, 200, 0);
         cvtColor(mat_part[i], mat_part[i],CV_GRAY2RGB);

         //I need draw only longest line, not borders!
         for( size_t j = 0; j < lines.size(); j++ )
         {
            if((lines[j][0] > 10)  || 
                (lines[j][0] < (mat_part[j].cols - 10)) || 
                (lines[j][2] > 10)  || 
                (lines[j][2] < (mat_part[j].cols - 10)))
            {
               continue;
            }

            int dy = abs(lines[j][1] - lines[j][3]);
            if (dy > dy_max)
            {
               dy_max = dy;
               l = lines[j];
            }
         }
         //draw longest
         line(mat_part[i],Point(l[0],l[1]),Point(l[2],l[3]),Scalar(255,0,0),1);
      }
      cvtColor(mat_part_mid, mat_part_mid,CV_GRAY2RGB);

      //middle horizontal line (need visible horizontal center of image)
      line(mat_part_mid,Point(0,mat_part_mid.rows/2),Point(mat_part_mid.cols,mat_part_mid.rows/2),
             Scalar(255,0,255),1);

      //pic reconstruction
      hconcat(mat_part[0],mat_part[1],mat_part_top);
      hconcat(mat_part[2],mat_part[3],mat_part_bot);
      vconcat(mat_part_top, mat_part_mid, mat_part_tm);
      vconcat(mat_part_tm, mat_part_bot, mat_part_whole);

      cvtColor(mat_part_whole,mat_canny_view,CV_GRAY2RGB);
      cv::resize(mat_canny_view,mat_canny_view_s,sizeMat);
      QImage qimgProcCanny((uchar*)mat_canny_view_s.data, mat_canny_view_s.cols, mat_canny_view_s.rows,    
                                           mat_canny_view_s.step, QImage::Format_RGB888);
      ui->label_result->setPixmap(QPixmap::fromImage(qimgProcCanny));