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 contour1, std::vector contour2 ) {
    double i = fabs( contourArea(cv::Mat(contour1)) );
    double j = fabs( contourArea(cv::Mat(contour2)) );
    return ( i < j );
}

Usage:

[...]

// find contours
std::vector > contours;
std::vector 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 biggestContour = contours[contours.size()-1];
std::vector smallestContour = contours[0];