用实例分析opencv查找连通区域最大面积

#include <opencv2/imgproc.hpp>
#include <opencv2/highgui.hpp>
 
using namespace cv;
using namespace std;
 
int main( int argc, char** argv )
{
  Mat src = imread( argv[1] );
 
  int largest_area=0;
  int largest_contour_index=0;
  Rect bounding_rect;
 
  Mat thr;
  cvtColor( src, thr, COLOR_BGR2GRAY ); //Convert to gray
  threshold( thr, thr, 125, 255, THRESH_BINARY ); //Threshold the gray
  bitwise_not(thr,thr); //这里先变反转颜色
 
  vector<vector<Point> > contours; // Vector for storing contours
 
  findContours( thr, contours, RETR_CCOMP, CHAIN_APPROX_SIMPLE ); // Find the contours in the image
 
  for( size_t i = 0; i< contours.size(); i++ ) // iterate through each contour.
  {
    double area = contourArea( contours[i] ); // Find the area of contour
 
    if( area > largest_area )
    {
      largest_area = area;
      largest_contour_index = i;        //Store the index of largest contour
      bounding_rect = boundingRect( contours[i] ); // Find the bounding rectangle for biggest contour
    }
  }
 
  drawContours( src, contours,largest_contour_index, Scalar( 0, 255, 0 ), 2 ); // Draw the largest contour using previously stored index.
 
  imshow( "result", src );
  waitKey();
  return 0;
}

 std::pair< int , int > MaxAreaFromSource(Mat srcImage, Mat &dstImage, int index)
{
  /*
  vector<vector<cv::Point> > contours; // Vector for storing contours
  
  int largest_area=0;
  size_t largest_contour_index=0;
  Rect bounding_rect;
  
  findContours( srcImage, contours, RETR_CCOMP, CHAIN_APPROX_SIMPLE ); // Find the contours in the image
  
  for( size_t i = 0; i< contours.size(); i++ ) // iterate through each contour.
  {
    double area = contourArea( contours[i] ); // Find the area of contour
    
    if( area > largest_area )
    {
      largest_area = area;
      largest_contour_index = i;        //Store the index of largest contour
      bounding_rect = boundingRect( contours[i] ); // Find the bounding rectangle for biggest contour
    }
  }
  
  Mat dst;
  cvtColor(srcImage, dst, CV_GRAY2RGB);
  drawContours( dst, contours,largest_contour_index, Scalar( 0, 255, 0 ), 2 ); // Draw the largest contour using previously stored index.
  imshow( "result", dst );
  waitKey();
  
  printf("%%%%%%%%%%%max area:%d\n", largest_area);
  return make_pair( largest_area, index);
  */
  
  cv::Mat img_bool, labels, stats, centroids, img_color, img_gray;
  
  //连通域计算
  int nccomps = cv::connectedComponentsWithStats (
                          srcImage, //二值图像
                          labels,   //和原图一样大的标记图
                          stats, //nccomps×5的矩阵 表示每个连通区域的外接矩形和面积（pixel）
                          centroids //nccomps×2的矩阵 表示每个连通区域的质心
                          );
  //cv::imshow("labels", labels);
  //cv::waitKey();
  
  vector<cv::Vec3b> colors(nccomps);
  colors[0] = cv::Vec3b(0,0,0); // background pixels remain black.
  
   printf( "index:%d==================\n",index );
  
  vector< int >vec_width,vec_area,vec_height;
  
  for(int label = 1; label < nccomps; ++label)
  {
    colors[label] = cv::Vec3b( (std::rand()&255), (std::rand()&255), (std::rand()&255) );
    std::cout << "Component "<< label << std::endl;
    std::cout << "CC_STAT_LEFT  = " << stats.at<int>(label,cv::CC_STAT_LEFT) << std::endl;
    std::cout << "CC_STAT_TOP  = " << stats.at<int>(label,cv::CC_STAT_TOP) << std::endl;
    std::cout << "CC_STAT_WIDTH = " << stats.at<int>(label,cv::CC_STAT_WIDTH) << std::endl;
    std::cout << "CC_STAT_HEIGHT = " << stats.at<int>(label,cv::CC_STAT_HEIGHT) << std::endl;
    std::cout << "CC_STAT_AREA  = " << stats.at<int>(label,cv::CC_STAT_AREA) << std::endl;
    std::cout << "CENTER  = (" << centroids.at<double>(label, 0) <<","<< centroids.at<double>(label, 1) << ")"<< std::endl << std::endl;
    
    int area = stats.at<int>(label,cv::CC_STAT_AREA);
    int left = stats.at<int>(label,cv::CC_STAT_LEFT);
    int top = stats.at<int>(label,cv::CC_STAT_TOP);
    int width = stats.at<int>(label,cv::CC_STAT_WIDTH);
    int height = stats.at<int>(label,cv::CC_STAT_HEIGHT);
    
    vec_area.push_back(area);
    vec_width.push_back(width);
    vec_height.push_back(height);
  }
  
  vector<int>::iterator bigwidth = std::max_element(std::begin(vec_width), std::end(vec_width));
  vector<int>::iterator bigheight = std::max_element(std::begin(vec_height), std::end(vec_height));
  vector<int>::iterator bigarea = std::max_element(std::begin(vec_area), std::end(vec_area));
  
  //printf( "area:%d------------width:%d height:%d \n", *bigarea, *bigwidth, *bigheight );
  
  //按照label值，对不同的连通域进行着色
  img_color = cv::Mat::zeros(srcImage.size(), CV_8UC3);
  for( int y = 0; y < img_color.rows; y++ )
    for( int x = 0; x < img_color.cols; x++ )
    {
      int label = labels.at<int>(y, x);
      CV_Assert(0 <= label && label <= nccomps);
      img_color.at<cv::Vec3b>(y, x) = colors[label];
    }
  
  cv::imshow("color", img_color);
  cv::waitKey();
   
  return make_pair( *bigarea , index );
}