Subtract all contours from image except the one with the largest area - c++

Question

cv::Mat thr;
std::vector<std::vector<cv::Point> > contours;
std::vector<std::vector<cv::Vec4i> > hierarchy;
int largest_area = 0;
int largest_contour_index = 0;
cv::findContours( thr, contours, hierarchy,CV_RETR_CCOMP, CV_CHAIN_APPROX_SIMPLE ); // Find the contours in the image
for( int i = 0; i < contours.size(); i++ ) // iterate through each contour.
{
double a = contourArea( contours[i], false ); // Find the area of contour
if(a > largest_area)
{
largest_area = a;
largest_contour_index = i; // Store the index of largest contour
}
}
What should I do after finding the index of the largest contour? How I can delete all the other contours with its inner areas?
Image is binary (cv::Mat thr). Just black background with white areas.
Thanks.

Answer 1

In your case, deleting contours with its inner areas is equal to fill them to black. This can be done by drawing the contour regions with black color:
for (size_t i=0; i<contours.size(); ++i) {
if (i != largest_contour_index) { // not the largest one
cv::drawContours(thr, contours, i, cv::Scalar(0,0,0), CV_FILLED);
}
}

Answer 2

After finding contours find index of biggest contour and draw that contour on Mat.
int indexOfBiggestContour = -1;
int sizeOfBiggestContour = 0;
for (int i = 0; i < contours.size(); i++)
{
if (contours[i].size() > sizeOfBiggestContour)
{
sizeOfBiggestContour = contours[i].size();
indexOfBiggestContour = i;
}
}
cv::Mat newImage;
drawContours(newImage, contours, indexOfBiggestContour, Scalar(255), CV_FILLED, 8, hierarchy);