counting the number of child contours inside a contour in opencv - c++

Question

I have this source image
and I have applied binary thresholding to get this
I used contours to differentiate between the ones having child contours and ones that don't.The resultant pic is
But how do I count the number of child contours that each green contour contains?. This is code I have used:-
Mat binMask = lung;// the thresholded image
Mat lung_src = imread("source.tiff");// the source image
//imshow("bin mask", binMask);
vector<std::vector<cv::Point>> contours;
vector<cv::Vec4i> hierarchy;
int count = 0, j;
double largest_area = 0;
int largest_contour_index = 0;
findContours(binMask, contours, hierarchy, CV_RETR_TREE, CV_CHAIN_APPROX_SIMPLE, cv::Point(0, 0));
for (int i = 0; i < contours.size(); i++)
{
double a = contourArea(contours[i], false); // Find the area of contour
if (a>largest_area)
{
largest_area = a;
largest_contour_index = i;
}
for (j = 0; j <= i; j++)
{
if (hierarchy[j][2] != -1) // means it has child contour
{
drawContours(lung_src, contours, j, Scalar(0, 255, 0), 1, 8, hierarchy, 0, Point());
}
else // means it doesn't have any child contour
{
drawContours(lung_src, contours, j, Scalar(0, 0, 255), 1, 8, hierarchy, 0, Point());
}
}
}
drawContours(lung_src, contours, largest_contour_index, Scalar(255, 0, 0), 1, 8, hierarchy, 0, Point());
imshow("lung-mapped", lung_src);
EDIT-1- I added the code from Humam at the end to check it out:
std::vector<int> number_of_inner_contours(contours.size(), -1);
int number_of_childs = 0;
for (size_t i = 0; i < contours.size(); i++)
{
int first_child_index = hierarchy[i][2];
if (first_child_index >= 0)
{
int next_child_index = hierarchy[first_child_index][0];
if (number_of_inner_contours[next_child_index]<0)
{
number_of_childs = number_of_inner_contours[next_child_index];
}
else
{
while (next_child_index >= 0)
{
next_child_index = hierarchy[next_child_index][0];
++number_of_childs;
}
number_of_inner_contours[i] = number_of_childs;
}
}
else
{
number_of_inner_contours[i] = 0;
}
cout << "\nThe contour[" << i << "] has " << number_of_inner_contours[i] << "child contours";
}
But the output I got was like :
The contour[456 ] has 0 child contours
The contour[457 ] has 0 child contours
The contour[458 ] has 0 child contours
The contour[459 ] has -1 child contours

Answer 1

From OpenCV documentation :
hierarchy – Optional output vector, containing information about the
image topology. It has as many elements as the number of contours. For
each i-th contour contours[i] , the elements hierarchy[i][0] ,
hiearchyi , hiearchy[i][2] , and hiearchy[i][3] are set to
0-based indices in contours of the next and previous contours at the
same hierarchical level, the first child contour and the parent
contour, respectively. If for the contour i there are no next,
previous, parent, or nested contours, the corresponding elements of
hierarchy[i] will be negative.
This is untested code for doing the job:
std::vector<size_t> number_of_inner_contours;
number_of_inner_contours.reserve(contours.size());
for (size_t i = 0; i < contours.size(); i++){
size_t number_of_childs = 0;
auto first_child_index=hierarchy[i][2];
if(first_child_index>=0){
auto next_child_index=hierarchy[first_child_index][0];
while (next_child_index>=0){
next_child_index=hierarchy[next_child_index][0];
++number_of_childs;
}
number_of_inner_contours.emplace_back(number_of_childs);
}
else{
number_of_inner_contours.emplace_back(0);
}
}
This code could be done in a better way by using the concept of dynamic programming. This is a first try also:
std::vector<int> number_of_inner_contours(contours.size(),-1);
for (size_t i = 0; i < contours.size(); i++){
auto number_of_childs = 0;
auto first_child_index=hierarchy[i][2];
if(first_child_index>=0){
auto next_child_index=hierarchy[first_child_index][0];
if(number_of_inner_contours[next_child_index]<0){
number_of_childs=number_of_inner_contours[next_child_index];
}
else{
while (next_child_index>=0){
next_child_index=hierarchy[next_child_index][0];
++number_of_childs;
}
number_of_inner_contours[i]=number_of_childs;
}
}
else{
number_of_inner_contours[i]=0;
}
}