OpenCV Drawn Lines on Contour (c++)

Question

I want to draw lines on the following picture, that I can caclulate the length of each line. My problem is that when I try it with the following code my image get completly white.

 std::vector<cv::Vec2f> lines;
 cv::HoughLines(drawing_small, lines, 1, CV_PI/180, 50, 0, 0 );

 for( size_t i = 0; i < lines.size(); i++ )
 {
    float rho = lines[i][0], theta = lines[i][1];
    cv::Point pt1, pt2;
    double a = cos(theta), b = sin(theta);
    double x0 = a*rho, y0 = b*rho;
    pt1.x = cvRound(x0 + 1000*(-b));
    pt1.y = cvRound(y0 + 1000*(a));
    pt2.x = cvRound(x0 - 1000*(-b));
    pt2.y = cvRound(y0 - 1000*(a));
    cv::line( drawing_small, pt1, pt2, cv::Scalar(0,100,0), 3, CV_AA);
 }

Something like that:

I would be very happy if anyone can say me what I can do.

Update

This is what I do before:

cv::findContours(dst, contours_small, hierarchy_small, CV_RETR_TREE, CV_CHAIN_APPROX_SIMPLE, cv::Point(0, 0) );
//Detecting Contours
std::vector<cv::Point2f> ContCenter_small(contours_small.size());
cv::Mat drawing_small = cv::Mat::zeros( dst.size(), CV_8UC3 );
for( int i = 0; i < contours_small.size(); i++ )
{
    ContArea_small[i] = moments(contours_small[i], false);
    ContCenter_small[i] = cv::Point2f(ContArea_small[i].m10/ContArea_small[i].m00, ContArea_small[i].m01/ContArea_small[i].m00);
    cv::Scalar color_small = cv::Scalar(0,255,0);
    if(ContArea_small[i].m00 > 2000)
    {
    drawContours( drawing_small, contours_small, i, color_small, CV_FILLED , 8, hierarchy_small, 1, cv::Point() );
    }
}
cv::imwrite("contour.jpg",drawing_small);
cv::dilate(drawing_small, drawing_small,C,cv::Point(-1,-1),1,1,20);
cv::threshold(drawing_small,drawing_small,100,255,cv::THRESH_BINARY_INV);
cv::GaussianBlur(drawing_small,drawing_small,cv::Size(9,9),11);

Answer 1

This probably means that Hough Transform did manage to find any lines on your picture. In this case you should pre-filter your image first. For example, you can try Otsu's thresholding and Gaussian blur. And if I were you than I would first start from trying to pass different parameters to cv::HoughLines (especially threshold -- The minimum number of intersections to “detect” a line)

Answer 2

Make sure you are drawing lines on and outputting the source image instead of some processed one. Can you show us more code about what you did exactly.