OpenCV C++: Sorting contours by their contourArea

Question

How can I sort contours by the size of their contour areas? And how can I get the biggest/smallest one?

Answer 1

You can use std::sort with a custom comparison function object

// comparison function object
bool compareContourAreas ( std::vector<cv::Point> contour1, std::vector<cv::Point> contour2 ) {
    double i = fabs( contourArea(cv::Mat(contour1)) );
    double j = fabs( contourArea(cv::Mat(contour2)) );
    return ( i < j );
}

Usage:

[...]

// find contours
std::vector<std::vector<cv::Point> > contours;
std::vector<cv::Vec4i> hierarchy;
cv::findContours( binary_image, contours, hierarchy, CV_RETR_EXTERNAL, CV_CHAIN_APPROX_SIMPLE, cv::Point(0, 0) );

// sort contours
std::sort(contours.begin(), contours.end(), compareContourAreas);

// grab contours
std::vector<cv::Point> biggestContour = contours[contours.size()-1];
std::vector<cv::Point> smallestContour = contours[0];

Answer 2

Just give a solution using the lambda function if C++11 is available.

    sort(contours.begin(), contours.end(), [](const vector<Point>& c1, const vector<Point>& c2){
    return contourArea(c1, false) < contourArea(c2, false);
});

Then you can access contours[0] to get the contour with the smallest area and contours[contours.size()-1] to get the one with the largest area because the contours are sorted in ascending order.