最大輪郭を取得します.

opencv 3画像中の輪郭の面積を計算します.http://blog.csdn.net/qq_23880193/articale/detail/4925999
http://blog.csdn.net/davebobo/article/details/52583167

#include    
#include       
#include    
#include    

using namespace std;  
using namespace cv;  

int main(){  

    Mat image = cvLoadImage("group.jpg");  
    Mat grayImage;  
    cvtColor(image, grayImage, CV_BGR2GRAY);  
    //          
    Mat binaryImage;  
    threshold(grayImage, binaryImage, 90, 255, CV_THRESH_BINARY);  

    //             ，       255      （  ）， 0          
    Mat reverseBinaryImage;  
    bitwise_not(binaryImage, reverseBinaryImage);  

    vector <vector>contours;  
    findContours(reverseBinaryImage,  
        contours,   //       
        CV_RETR_EXTERNAL,   //       
        CV_CHAIN_APPROX_NONE);  //             
    //              
    Mat result(reverseBinaryImage.size(), CV_8U, Scalar(255));  
    drawContours(result, contours,  
        -1, //        
        Scalar(0),  //       
        2); //         2  

    namedWindow("contours");  
    imshow("contours", result);  

    //            
    int cmin = 100; //        
    int cmax = 1000;    //      
    vector<vector>::const_iterator itc = contours.begin();  
    while (itc!=contours.end())  
    {  
        if (itc->size() < cmin || itc->size() > cmax)  
            itc = contours.erase(itc);  
        else  
            ++itc;  
    }  

    //              
    Mat result_erase(binaryImage.size(), CV_8U, Scalar(255));  
    drawContours(result_erase, contours,  
        -1, //        
        Scalar(0),  //       
        2); //         2  

    namedWindow("contours_erase");  
    imshow("contours_erase", result_erase);  
    waitKey(0);  
    return 0;  
}