findContours
API of OpenCV modifies the image which it uses to find contours.You can try the following.
Mat displayContours = Mat::zeros(image.rows,image.cols,CV_8UC1);
for(int i = 1;i<(int)largest_contours[0].size();i++)
{
line(displayContours,largest_contours[0][i-1],largest_contours[0][i],255,2,8,0);
line(image,largest_contours[0][i-1],largest_contours[0][i],cv::Scalar(0,0,255),2,8,0);
}
Now display the displayContours
instead of foreground