opencv的確定4條最外角線從矩形對象

我是新來的OpenCV，我使用此代碼找到4個最外線爲了獲得角落的圖像中的矩形對象。opencv的確定4條最外角線從矩形對象

cv::Mat input(toOcv(surface)); 
cv::Mat output; 

cv::medianBlur(input, output, 21); 
Mat dst, cdst; 
Canny(input, dst, 50, 200, 3); 
cvtColor(dst, cdst, CV_GRAY2BGR); 

//Standard Hough 
vector<Vec2f> lines; 
HoughLines(dst, lines, 1, CV_PI/180, 100, 0, 0); 

// Draw the lines 
for (size_t i = 0; i < lines.size(); i++) 
{ 
    float rho = lines[i][0], theta = lines[i][1]; 
    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)); 

    line(cdst, pt1, pt2, Scalar(rand() % 255, rand() % 255, rand() % 255), 2, CV_AA);

}