求多边形或轮廓的凸包（Hull）

在图像处理和分析中，通常会遇到求物体的凸包或某类型形状的凸包，以下函数可以实现该功能：

void get_hull(cv::Mat& src,int thresh)
{
 Mat src_copy = src.clone();
 Mat threshold_output;
 vector<vector<Point> > contours;
 vector<Vec4i> hierarchy;
 /// Detect edges using Threshold
 threshold( src_gray, threshold_output, thresh, 255, THRESH_BINARY );
 /// Find contours  
 findContours( threshold_output, contours, hierarchy, CV_RETR_TREE, CV_CHAIN_APPROX_SIMPLE, Point(0, 0) );
 /// Find the convex hull object for each contour
 vector<vector<Point> >hull( contours.size() );
 for( int i = 0; i < contours.size(); i++ )
 {   convexHull( Mat(contours[i]), hull[i], false ); } 
 /// Draw contours + hull results
 Mat drawing = Mat::zeros( threshold_output.size(), CV_8UC3 );
 for( int i = 0; i< contours.size(); i++ )
 { 
  Scalar color = Scalar( rng.uniform(0, 255), rng.uniform(0,255), rng.uniform(0,255) );
  drawContours( drawing, contours, i, color, 1, 8, vector<Vec4i>(), 0, Point() ); 
  drawContours( drawing, hull, i, color, 1, 8, vector<Vec4i>(), 0, Point() );       
 }
}