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my solution is

threshold(mat_part[i], mat_part[i], 0, 255, CV_THRESH_BINARY + CV_THRESH_OTSU);
findContours(mat_part[i],contour,CV_RETR_TREE,CV_CHAIN_APPROX_SIMPLE);

for(k = 0; k < contour.size(); k++)
         {
            ///skip contours with smaller size than specified area
            double area = contourArea(contour[k]);
            if (area > area_max)
            {
             area_max = area;
             index_of_area_max = k;
            }
         }
for(size_t j = 0; j < contour[index_of_area_max].size();j++) // run until j < contours[i].size();
         {
            pold = pixel;
            pixel = Point(contour[index_of_area_max][j]); //do whatever
            if (pixel.x > 10 && pixel.x < (max_x-10))
            {
               line_to_draw.push_back(pixel);
            }

aproximation to line with  my own function
print out

Thanks Folks!